How to handle calendar TimeZones using Java

Question

I have a Timestamp value that comes from my application  The user can be in any given local TimeZone   Since this date is used for a WebService that assumes the time given is always in GMT  I have a need to convert the user s parameter from say  EST  to  GMT   Here s the kicker  The user is oblivious to his TZ  He enters the creation date that he wants to send to the WS  so what I need is   User enters  5 1 2008 6 12 PM  EST   The parameter to the WS needs to be  5 1 2008 6 12 PM  GMT   I know TimeStamps are always supposed to be in GMT by default  but when sending the parameter  even though I created my Calendar from the TS  which is supposed to be in GMT   the hours are always off unless the user is in GMT  What am I missing   Timestamp issuedDate    Timestamp  getACPValue inputs    issuedDate    Calendar issueDate   convertTimestampToJavaCalendar issuedDate       private static java util Calendar convertTimestampToJavaCalendar Timestamp ts       java util Calendar cal   java util Calendar getInstance        GMT TIMEZONE  EN US LOCALE     cal setTimeInMillis ts  getTime       return cal      With the previous Code  this is what I get as a result  Short Format for easy reading     May 1  2008 11 12 PM

User · Answer

Something that has worked for me in the past was to determine the offset (in milliseconds) between the user's timezone and GMT. Once you have the offset, you can simply add/subtract (depending on which way the conversion is going) to get the appropriate time in either timezone. I would usually accomplish this by setting the milliseconds field of a Calendar object, but I'm sure you could easily apply it to a timestamp object. Here's the code I use to get the offset

int offset = TimeZone.getTimeZone(timezoneId).getRawOffset();

timezoneId is the id of the user's timezone (such as EST).

User · Answer

Method for converting from one timeZone to other probably it works               Adapt calendar to client time zone      param calendar - adapting calendar     param timeZone - client time zone     return adapt calendar to client time zone     public static Calendar convertCalendar final Calendar calendar  final TimeZone timeZone        Calendar ret   new GregorianCalendar timeZone       ret setTimeInMillis calendar getTimeInMillis                 timeZone getOffset calendar getTimeInMillis    -             TimeZone getDefault   getOffset calendar getTimeInMillis          ret getTime        return ret

User · Answer

java time  The modern approach uses the java time classes that supplanted the troublesome legacy date-time classes bundled with the earliest versions of Java   The java sql Timestamp class is one of those legacy classes  No longer needed  Instead use Instant or other java time classes directly with your database using JDBC 4 2 and later    The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds  up to nine  9  digits of a decimal fraction     Instant instant   myResultSet getObject        Instant class        If you must interoperate with an existing Timestamp  convert immediately into java time via the new conversion methods added to the old classes   Instant instant   myTimestamp toInstant       To adjust into another time zone  specify the time zone as a ZoneId object  Specify a proper time zone name in the format of continent region  such as America Montreal  Africa Casablanca  or Pacific Auckland  Never use the 3-4 letter pseudo-zones such as EST or IST as they are not true time zones  not standardized  and not even unique       ZoneId z   ZoneId of   America Montreal        Apply to the Instant to produce a ZonedDateTime object   ZonedDateTime zdt   instant atZone  z       To generate a string for display to the user  search Stack Overflow for DateTimeFormatter to find many discussions and examples   Your Question is really about going the other direction  from user data-entry to the date-time objects  Generally best to break your data-entry into two parts  a date and a time-of-day   LocalDate ld   LocalDate parse  dateInput   DateTimeFormatter ofPattern   M d uuuu    Locale US       LocalTime lt   LocalTime parse  timeInput   DateTimeFormatter ofPattern   H m a    Locale US         Your Question is not clear  Do you want to interpret the date and the time entered by the user to be in UTC  Or in another time zone   If you meant UTC  create a OffsetDateTime with an offset using the constant for UTC  ZoneOffset UTC    OffsetDateTime odt   OffsetDateTime of  ld   lt   ZoneOffset UTC       If you meant another time zone  combine along with a time zone object  a ZoneId  But which time zone  You might detect a default time zone  Or  if critical  you must confirm with the user to be certain of their intention   ZonedDateTime zdt   ZonedDateTime of  ld   lt   z       To get a simpler object that is always in UTC by definition  extract an Instant   Instant instant   odt toInstant          or     Instant instant   zdt toInstant        Send to your database   myPreparedStatement setObject        instant         About java time  The java time framework is built into Java 8 and later  These classes supplant the troublesome old legacy date-time classes such as java util Date  Calendar   amp  SimpleDateFormat   The Joda-Time project  now in maintenance mode  advises migration to the java time classes   To learn more  see the Oracle Tutorial  And search Stack Overflow for many examples and explanations  Specification is JSR 310   Where to obtain the java time classes     Java SE 8  Java SE 9  and later   Built-in   Part of the standard Java API with a bundled implementation  Java 9 adds some minor features and fixes   Java SE 6 and Java SE 7   Much of the java time functionality is back-ported to Java 6  amp  7 in ThreeTen-Backport   Android   Later versions of Android bundle implementations of the java time classes  For earlier Android  the ThreeTenABP project adapts ThreeTen-Backport  mentioned above   See How to use ThreeTenABP        The ThreeTen-Extra project extends java time with additional classes  This project is a proving ground for possible future additions to java time  You may find some useful classes here such as Interval  YearWeek  YearQuarter  and more

User · Answer

You say that the date is used in connection with web services  so I assume that is serialized into a string at some point   If this is the case  you should take a look at the setTimeZone method of the DateFormat class  This dictates which time zone that will be used when printing the time stamp   A simple example    SimpleDateFormat formatter   new SimpleDateFormat  yyyy-MM-dd T HH mm ss SSS Z     formatter setTimeZone TimeZone getTimeZone  UTC      Calendar cal   Calendar getInstance    String timestamp   formatter format cal getTime

User · Answer

Thank you all for responding  After a further investigation I got to the right answer  As mentioned by Skip Head  the TimeStamped I was getting from my application was being adjusted to the user s TimeZone  So if the User entered 6 12 PM  EST  I would get 2 12 PM  GMT   What I needed was a way to undo the conversion so that the time entered by the user is the time I sent to the WebServer request  Here s how I accomplished this      Get TimeZone of user TimeZone currentTimeZone   sc  getTimeZone    Calendar currentDt   new GregorianCalendar currentTimeZone  EN US LOCALE      Get the Offset from GMT taking DST into account int gmtOffset   currentTimeZone getOffset      currentDt get Calendar ERA        currentDt get Calendar YEAR        currentDt get Calendar MONTH        currentDt get Calendar DAY OF MONTH        currentDt get Calendar DAY OF WEEK        currentDt get Calendar MILLISECOND       convert to hours gmtOffset   gmtOffset    60 60 1000   System out println  Current User s TimeZone      currentTimeZone getID     System out println  Current Offset from GMT  in hrs      gmtOffset      Get TS from User Input Timestamp issuedDate    Timestamp  getACPValue inputs    issuedDate    System out println  TS from ACP      issuedDate      Set TS into Calendar Calendar issueDate   convertTimestampToJavaCalendar issuedDate      Adjust for GMT  note the offset negation  issueDate add Calendar HOUR OF DAY  -gmtOffset   System out println  Calendar Date converted from TS using GMT and US EN Locale          DateFormat getDateTimeInstance DateFormat SHORT  DateFormat SHORT       format issueDate getTime        The code s output is   User entered 5 1 2008 6 12PM  EST   Current User s TimeZone  EST Current Offset from GMT  in hrs  -4  Normally -5  except is DST adjusted  TS from ACP  2008-05-01 14 12 00 0 Calendar Date converted from TS using GMT and US EN Locale  5 1 08 6 12 PM  GMT

User · Answer

java time  The modern approach uses the java time classes that supplanted the troublesome legacy date-time classes bundled with the earliest versions of Java   The java sql Timestamp class is one of those legacy classes  No longer needed  Instead use Instant or other java time classes directly with your database using JDBC 4 2 and later    The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds  up to nine  9  digits of a decimal fraction     Instant instant   myResultSet getObject        Instant class        If you must interoperate with an existing Timestamp  convert immediately into java time via the new conversion methods added to the old classes   Instant instant   myTimestamp toInstant       To adjust into another time zone  specify the time zone as a ZoneId object  Specify a proper time zone name in the format of continent region  such as America Montreal  Africa Casablanca  or Pacific Auckland  Never use the 3-4 letter pseudo-zones such as EST or IST as they are not true time zones  not standardized  and not even unique       ZoneId z   ZoneId of   America Montreal        Apply to the Instant to produce a ZonedDateTime object   ZonedDateTime zdt   instant atZone  z       To generate a string for display to the user  search Stack Overflow for DateTimeFormatter to find many discussions and examples   Your Question is really about going the other direction  from user data-entry to the date-time objects  Generally best to break your data-entry into two parts  a date and a time-of-day   LocalDate ld   LocalDate parse  dateInput   DateTimeFormatter ofPattern   M d uuuu    Locale US       LocalTime lt   LocalTime parse  timeInput   DateTimeFormatter ofPattern   H m a    Locale US         Your Question is not clear  Do you want to interpret the date and the time entered by the user to be in UTC  Or in another time zone   If you meant UTC  create a OffsetDateTime with an offset using the constant for UTC  ZoneOffset UTC    OffsetDateTime odt   OffsetDateTime of  ld   lt   ZoneOffset UTC       If you meant another time zone  combine along with a time zone object  a ZoneId  But which time zone  You might detect a default time zone  Or  if critical  you must confirm with the user to be certain of their intention   ZonedDateTime zdt   ZonedDateTime of  ld   lt   z       To get a simpler object that is always in UTC by definition  extract an Instant   Instant instant   odt toInstant          or     Instant instant   zdt toInstant        Send to your database   myPreparedStatement setObject        instant         About java time  The java time framework is built into Java 8 and later  These classes supplant the troublesome old legacy date-time classes such as java util Date  Calendar   amp  SimpleDateFormat   The Joda-Time project  now in maintenance mode  advises migration to the java time classes   To learn more  see the Oracle Tutorial  And search Stack Overflow for many examples and explanations  Specification is JSR 310   Where to obtain the java time classes     Java SE 8  Java SE 9  and later   Built-in   Part of the standard Java API with a bundled implementation  Java 9 adds some minor features and fixes   Java SE 6 and Java SE 7   Much of the java time functionality is back-ported to Java 6  amp  7 in ThreeTen-Backport   Android   Later versions of Android bundle implementations of the java time classes  For earlier Android  the ThreeTenABP project adapts ThreeTen-Backport  mentioned above   See How to use ThreeTenABP        The ThreeTen-Extra project extends java time with additional classes  This project is a proving ground for possible future additions to java time  You may find some useful classes here such as Interval  YearWeek  YearQuarter  and more

User · Answer

Thank you all for responding  After a further investigation I got to the right answer  As mentioned by Skip Head  the TimeStamped I was getting from my application was being adjusted to the user s TimeZone  So if the User entered 6 12 PM  EST  I would get 2 12 PM  GMT   What I needed was a way to undo the conversion so that the time entered by the user is the time I sent to the WebServer request  Here s how I accomplished this      Get TimeZone of user TimeZone currentTimeZone   sc  getTimeZone    Calendar currentDt   new GregorianCalendar currentTimeZone  EN US LOCALE      Get the Offset from GMT taking DST into account int gmtOffset   currentTimeZone getOffset      currentDt get Calendar ERA        currentDt get Calendar YEAR        currentDt get Calendar MONTH        currentDt get Calendar DAY OF MONTH        currentDt get Calendar DAY OF WEEK        currentDt get Calendar MILLISECOND       convert to hours gmtOffset   gmtOffset    60 60 1000   System out println  Current User s TimeZone      currentTimeZone getID     System out println  Current Offset from GMT  in hrs      gmtOffset      Get TS from User Input Timestamp issuedDate    Timestamp  getACPValue inputs    issuedDate    System out println  TS from ACP      issuedDate      Set TS into Calendar Calendar issueDate   convertTimestampToJavaCalendar issuedDate      Adjust for GMT  note the offset negation  issueDate add Calendar HOUR OF DAY  -gmtOffset   System out println  Calendar Date converted from TS using GMT and US EN Locale          DateFormat getDateTimeInstance DateFormat SHORT  DateFormat SHORT       format issueDate getTime        The code s output is   User entered 5 1 2008 6 12PM  EST   Current User s TimeZone  EST Current Offset from GMT  in hrs  -4  Normally -5  except is DST adjusted  TS from ACP  2008-05-01 14 12 00 0 Calendar Date converted from TS using GMT and US EN Locale  5 1 08 6 12 PM  GMT

User · Answer

You can solve it with Joda Time   Date utcDate   new Date timezoneFrom convertLocalToUTC date getTime    false    Date localDate   new Date timezoneTo convertUTCToLocal utcDate getTime        Java 8   LocalDateTime localDateTime   LocalDateTime parse  2007-12-03T10 15 30    ZonedDateTime fromDateTime   localDateTime atZone      ZoneId of  America Toronto     ZonedDateTime toDateTime   fromDateTime withZoneSameInstant      ZoneId of  Canada Newfoundland

User · Answer

Thank you all for responding  After a further investigation I got to the right answer  As mentioned by Skip Head  the TimeStamped I was getting from my application was being adjusted to the user s TimeZone  So if the User entered 6 12 PM  EST  I would get 2 12 PM  GMT   What I needed was a way to undo the conversion so that the time entered by the user is the time I sent to the WebServer request  Here s how I accomplished this      Get TimeZone of user TimeZone currentTimeZone   sc  getTimeZone    Calendar currentDt   new GregorianCalendar currentTimeZone  EN US LOCALE      Get the Offset from GMT taking DST into account int gmtOffset   currentTimeZone getOffset      currentDt get Calendar ERA        currentDt get Calendar YEAR        currentDt get Calendar MONTH        currentDt get Calendar DAY OF MONTH        currentDt get Calendar DAY OF WEEK        currentDt get Calendar MILLISECOND       convert to hours gmtOffset   gmtOffset    60 60 1000   System out println  Current User s TimeZone      currentTimeZone getID     System out println  Current Offset from GMT  in hrs      gmtOffset      Get TS from User Input Timestamp issuedDate    Timestamp  getACPValue inputs    issuedDate    System out println  TS from ACP      issuedDate      Set TS into Calendar Calendar issueDate   convertTimestampToJavaCalendar issuedDate      Adjust for GMT  note the offset negation  issueDate add Calendar HOUR OF DAY  -gmtOffset   System out println  Calendar Date converted from TS using GMT and US EN Locale          DateFormat getDateTimeInstance DateFormat SHORT  DateFormat SHORT       format issueDate getTime        The code s output is   User entered 5 1 2008 6 12PM  EST   Current User s TimeZone  EST Current Offset from GMT  in hrs  -4  Normally -5  except is DST adjusted  TS from ACP  2008-05-01 14 12 00 0 Calendar Date converted from TS using GMT and US EN Locale  5 1 08 6 12 PM  GMT

User · Answer

You say that the date is used in connection with web services  so I assume that is serialized into a string at some point   If this is the case  you should take a look at the setTimeZone method of the DateFormat class  This dictates which time zone that will be used when printing the time stamp   A simple example    SimpleDateFormat formatter   new SimpleDateFormat  yyyy-MM-dd T HH mm ss SSS Z     formatter setTimeZone TimeZone getTimeZone  UTC      Calendar cal   Calendar getInstance    String timestamp   formatter format cal getTime

User · Answer

It looks like your TimeStamp is being set to the timezone of the originating system   This is deprecated  but it should work   cal setTimeInMillis ts  getTime   - ts  getTimezoneOffset       The non-deprecated way is to use  Calendar get Calendar ZONE OFFSET    Calendar get Calendar DST OFFSET      60   1000    but that would need to be done on the client side  since that system knows what timezone it is in

User · Answer

Something that has worked for me in the past was to determine the offset (in milliseconds) between the user's timezone and GMT. Once you have the offset, you can simply add/subtract (depending on which way the conversion is going) to get the appropriate time in either timezone. I would usually accomplish this by setting the milliseconds field of a Calendar object, but I'm sure you could easily apply it to a timestamp object. Here's the code I use to get the offset

int offset = TimeZone.getTimeZone(timezoneId).getRawOffset();

timezoneId is the id of the user's timezone (such as EST).

User · Answer

It looks like your TimeStamp is being set to the timezone of the originating system   This is deprecated  but it should work   cal setTimeInMillis ts  getTime   - ts  getTimezoneOffset       The non-deprecated way is to use  Calendar get Calendar ZONE OFFSET    Calendar get Calendar DST OFFSET      60   1000    but that would need to be done on the client side  since that system knows what timezone it is in

User · Answer

public static Calendar convertToGmt Calendar cal         Date date   cal getTime        TimeZone tz   cal getTimeZone         log debug  input calendar has date      date                Returns the number of milliseconds since January 1  1970  00 00 00 GMT      long msFromEpochGmt   date getTime           gives you the current offset in ms from GMT at the current date     int offsetFromUTC   tz getOffset msFromEpochGmt       log debug  offset is     offsetFromUTC          create a new calendar in GMT timezone  set to this date and add the offset     Calendar gmtCal   Calendar getInstance TimeZone getTimeZone  GMT         gmtCal setTime date       gmtCal add Calendar MILLISECOND  offsetFromUTC        log debug  Created GMT cal with date      gmtCal getTime                return gmtCal      Here s the output if I pass the current time   12 09 05 EDT  from Calendar getInstance    in      DEBUG - input calendar has date  Thu Oct 23 12 09 05 EDT 2008    DEBUG - offset is -14400000   DEBUG - Created GMT cal with date  Thu Oct 23 08 09 05 EDT 2008    12 09 05 GMT is 8 09 05 EDT   The confusing part here is that Calendar getTime   returns you a Date in your current timezone  and also that there is no method to modify the timezone of a calendar and have the underlying date rolled also  Depending on what type of parameter your web service takes  your may just want to have the WS deal in terms of milliseconds from epoch

User · Answer

Date and Timestamp objects are timezone-oblivious  they represent a certain number of seconds since the epoch  without committing to a particular interpretation of that instant as hours and days   Timezones enter the picture only in GregorianCalendar  not directly needed for this task  and SimpleDateFormat  which need a timezone offset to convert between separate fields and Date  or long  values   The OP s problem is right at the beginning of his processing  the user inputs hours  which are ambiguous  and they are interpreted in the local  non-GMT timezone  at this point the value is  6 12 EST   which can be easily printed as  11 12 GMT  or any other timezone but is never going to change to  6 12 GMT    There is no way to make the SimpleDateFormat that parses  06 12  as  HH MM   defaulting to the local time zone  default to UTC instead  SimpleDateFormat is a bit too smart for its own good   However  you can convince any SimpleDateFormat instance to use the right time zone if you put it explicitly in the input  just append a fixed string to the received  and adequately validated   06 12  to parse  06 12 GMT  as  HH MM z     There is no need of explicit setting of GregorianCalendar fields or of retrieving and using timezone and daylight saving time offsets   The real problem is segregating inputs that default to the local timezone  inputs that default to UTC  and inputs that really require an explicit timezone indication

User · Answer

It looks like your TimeStamp is being set to the timezone of the originating system   This is deprecated  but it should work   cal setTimeInMillis ts  getTime   - ts  getTimezoneOffset       The non-deprecated way is to use  Calendar get Calendar ZONE OFFSET    Calendar get Calendar DST OFFSET      60   1000    but that would need to be done on the client side  since that system knows what timezone it is in

User · Answer

public static Calendar convertToGmt Calendar cal         Date date   cal getTime        TimeZone tz   cal getTimeZone         log debug  input calendar has date      date                Returns the number of milliseconds since January 1  1970  00 00 00 GMT      long msFromEpochGmt   date getTime           gives you the current offset in ms from GMT at the current date     int offsetFromUTC   tz getOffset msFromEpochGmt       log debug  offset is     offsetFromUTC          create a new calendar in GMT timezone  set to this date and add the offset     Calendar gmtCal   Calendar getInstance TimeZone getTimeZone  GMT         gmtCal setTime date       gmtCal add Calendar MILLISECOND  offsetFromUTC        log debug  Created GMT cal with date      gmtCal getTime                return gmtCal      Here s the output if I pass the current time   12 09 05 EDT  from Calendar getInstance    in      DEBUG - input calendar has date  Thu Oct 23 12 09 05 EDT 2008    DEBUG - offset is -14400000   DEBUG - Created GMT cal with date  Thu Oct 23 08 09 05 EDT 2008    12 09 05 GMT is 8 09 05 EDT   The confusing part here is that Calendar getTime   returns you a Date in your current timezone  and also that there is no method to modify the timezone of a calendar and have the underlying date rolled also  Depending on what type of parameter your web service takes  your may just want to have the WS deal in terms of milliseconds from epoch

User · Answer

Something that has worked for me in the past was to determine the offset (in milliseconds) between the user's timezone and GMT. Once you have the offset, you can simply add/subtract (depending on which way the conversion is going) to get the appropriate time in either timezone. I would usually accomplish this by setting the milliseconds field of a Calendar object, but I'm sure you could easily apply it to a timestamp object. Here's the code I use to get the offset

int offset = TimeZone.getTimeZone(timezoneId).getRawOffset();

timezoneId is the id of the user's timezone (such as EST).

User · Answer

You say that the date is used in connection with web services  so I assume that is serialized into a string at some point   If this is the case  you should take a look at the setTimeZone method of the DateFormat class  This dictates which time zone that will be used when printing the time stamp   A simple example    SimpleDateFormat formatter   new SimpleDateFormat  yyyy-MM-dd T HH mm ss SSS Z     formatter setTimeZone TimeZone getTimeZone  UTC      Calendar cal   Calendar getInstance    String timestamp   formatter format cal getTime

User · Answer

You say that the date is used in connection with web services  so I assume that is serialized into a string at some point   If this is the case  you should take a look at the setTimeZone method of the DateFormat class  This dictates which time zone that will be used when printing the time stamp   A simple example    SimpleDateFormat formatter   new SimpleDateFormat  yyyy-MM-dd T HH mm ss SSS Z     formatter setTimeZone TimeZone getTimeZone  UTC      Calendar cal   Calendar getInstance    String timestamp   formatter format cal getTime

User · Answer

Date and Timestamp objects are timezone-oblivious  they represent a certain number of seconds since the epoch  without committing to a particular interpretation of that instant as hours and days   Timezones enter the picture only in GregorianCalendar  not directly needed for this task  and SimpleDateFormat  which need a timezone offset to convert between separate fields and Date  or long  values   The OP s problem is right at the beginning of his processing  the user inputs hours  which are ambiguous  and they are interpreted in the local  non-GMT timezone  at this point the value is  6 12 EST   which can be easily printed as  11 12 GMT  or any other timezone but is never going to change to  6 12 GMT    There is no way to make the SimpleDateFormat that parses  06 12  as  HH MM   defaulting to the local time zone  default to UTC instead  SimpleDateFormat is a bit too smart for its own good   However  you can convince any SimpleDateFormat instance to use the right time zone if you put it explicitly in the input  just append a fixed string to the received  and adequately validated   06 12  to parse  06 12 GMT  as  HH MM z     There is no need of explicit setting of GregorianCalendar fields or of retrieving and using timezone and daylight saving time offsets   The real problem is segregating inputs that default to the local timezone  inputs that default to UTC  and inputs that really require an explicit timezone indication

User · Answer

Thank you all for responding  After a further investigation I got to the right answer  As mentioned by Skip Head  the TimeStamped I was getting from my application was being adjusted to the user s TimeZone  So if the User entered 6 12 PM  EST  I would get 2 12 PM  GMT   What I needed was a way to undo the conversion so that the time entered by the user is the time I sent to the WebServer request  Here s how I accomplished this      Get TimeZone of user TimeZone currentTimeZone   sc  getTimeZone    Calendar currentDt   new GregorianCalendar currentTimeZone  EN US LOCALE      Get the Offset from GMT taking DST into account int gmtOffset   currentTimeZone getOffset      currentDt get Calendar ERA        currentDt get Calendar YEAR        currentDt get Calendar MONTH        currentDt get Calendar DAY OF MONTH        currentDt get Calendar DAY OF WEEK        currentDt get Calendar MILLISECOND       convert to hours gmtOffset   gmtOffset    60 60 1000   System out println  Current User s TimeZone      currentTimeZone getID     System out println  Current Offset from GMT  in hrs      gmtOffset      Get TS from User Input Timestamp issuedDate    Timestamp  getACPValue inputs    issuedDate    System out println  TS from ACP      issuedDate      Set TS into Calendar Calendar issueDate   convertTimestampToJavaCalendar issuedDate      Adjust for GMT  note the offset negation  issueDate add Calendar HOUR OF DAY  -gmtOffset   System out println  Calendar Date converted from TS using GMT and US EN Locale          DateFormat getDateTimeInstance DateFormat SHORT  DateFormat SHORT       format issueDate getTime        The code s output is   User entered 5 1 2008 6 12PM  EST   Current User s TimeZone  EST Current Offset from GMT  in hrs  -4  Normally -5  except is DST adjusted  TS from ACP  2008-05-01 14 12 00 0 Calendar Date converted from TS using GMT and US EN Locale  5 1 08 6 12 PM  GMT

User · Answer

Something that has worked for me in the past was to determine the offset (in milliseconds) between the user's timezone and GMT. Once you have the offset, you can simply add/subtract (depending on which way the conversion is going) to get the appropriate time in either timezone. I would usually accomplish this by setting the milliseconds field of a Calendar object, but I'm sure you could easily apply it to a timestamp object. Here's the code I use to get the offset

int offset = TimeZone.getTimeZone(timezoneId).getRawOffset();

timezoneId is the id of the user's timezone (such as EST).

User · Answer

public static Calendar convertToGmt Calendar cal         Date date   cal getTime        TimeZone tz   cal getTimeZone         log debug  input calendar has date      date                Returns the number of milliseconds since January 1  1970  00 00 00 GMT      long msFromEpochGmt   date getTime           gives you the current offset in ms from GMT at the current date     int offsetFromUTC   tz getOffset msFromEpochGmt       log debug  offset is     offsetFromUTC          create a new calendar in GMT timezone  set to this date and add the offset     Calendar gmtCal   Calendar getInstance TimeZone getTimeZone  GMT         gmtCal setTime date       gmtCal add Calendar MILLISECOND  offsetFromUTC        log debug  Created GMT cal with date      gmtCal getTime                return gmtCal      Here s the output if I pass the current time   12 09 05 EDT  from Calendar getInstance    in      DEBUG - input calendar has date  Thu Oct 23 12 09 05 EDT 2008    DEBUG - offset is -14400000   DEBUG - Created GMT cal with date  Thu Oct 23 08 09 05 EDT 2008    12 09 05 GMT is 8 09 05 EDT   The confusing part here is that Calendar getTime   returns you a Date in your current timezone  and also that there is no method to modify the timezone of a calendar and have the underlying date rolled also  Depending on what type of parameter your web service takes  your may just want to have the WS deal in terms of milliseconds from epoch

User · Answer

You can solve it with Joda Time   Date utcDate   new Date timezoneFrom convertLocalToUTC date getTime    false    Date localDate   new Date timezoneTo convertUTCToLocal utcDate getTime        Java 8   LocalDateTime localDateTime   LocalDateTime parse  2007-12-03T10 15 30    ZonedDateTime fromDateTime   localDateTime atZone      ZoneId of  America Toronto     ZonedDateTime toDateTime   fromDateTime withZoneSameInstant      ZoneId of  Canada Newfoundland

User · Answer

public static Calendar convertToGmt Calendar cal         Date date   cal getTime        TimeZone tz   cal getTimeZone         log debug  input calendar has date      date                Returns the number of milliseconds since January 1  1970  00 00 00 GMT      long msFromEpochGmt   date getTime           gives you the current offset in ms from GMT at the current date     int offsetFromUTC   tz getOffset msFromEpochGmt       log debug  offset is     offsetFromUTC          create a new calendar in GMT timezone  set to this date and add the offset     Calendar gmtCal   Calendar getInstance TimeZone getTimeZone  GMT         gmtCal setTime date       gmtCal add Calendar MILLISECOND  offsetFromUTC        log debug  Created GMT cal with date      gmtCal getTime                return gmtCal      Here s the output if I pass the current time   12 09 05 EDT  from Calendar getInstance    in      DEBUG - input calendar has date  Thu Oct 23 12 09 05 EDT 2008    DEBUG - offset is -14400000   DEBUG - Created GMT cal with date  Thu Oct 23 08 09 05 EDT 2008    12 09 05 GMT is 8 09 05 EDT   The confusing part here is that Calendar getTime   returns you a Date in your current timezone  and also that there is no method to modify the timezone of a calendar and have the underlying date rolled also  Depending on what type of parameter your web service takes  your may just want to have the WS deal in terms of milliseconds from epoch

User · Answer

Method for converting from one timeZone to other probably it works               Adapt calendar to client time zone      param calendar - adapting calendar     param timeZone - client time zone     return adapt calendar to client time zone     public static Calendar convertCalendar final Calendar calendar  final TimeZone timeZone        Calendar ret   new GregorianCalendar timeZone       ret setTimeInMillis calendar getTimeInMillis                 timeZone getOffset calendar getTimeInMillis    -             TimeZone getDefault   getOffset calendar getTimeInMillis          ret getTime        return ret

User · Answer

It looks like your TimeStamp is being set to the timezone of the originating system   This is deprecated  but it should work   cal setTimeInMillis ts  getTime   - ts  getTimezoneOffset       The non-deprecated way is to use  Calendar get Calendar ZONE OFFSET    Calendar get Calendar DST OFFSET      60   1000    but that would need to be done on the client side  since that system knows what timezone it is in

[java] How to handle calendar TimeZones using Java?

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