How to change TIMEZONE for a java util Calendar Date

Question

I would like to change the TIMEZONE value in a Java Calendar instance at runtime  I tried below  But the output is the same in both instances       Calendar cSchedStartCal   Calendar getInstance TimeZone getTimeZone  GMT         System out println cSchedStartCal getTime   getTime         cSchedStartCal setTimeZone TimeZone getTimeZone  Asia Calcutta         System out println cSchedStartCal getTime   getTime       OUTPUT   1353402486773  1353402486773  I have tried this also but the output is still the same       Calendar cSchedStartCal   Calendar getInstance TimeZone getTimeZone  GMT         System out println cSchedStartCal getTime          Calendar cSchedStartCal1   Calendar getInstance TimeZone getTimeZone  Asia Calcutta         cSchedStartCal1 setTime cSchedStartCal getTime         System out println cSchedStartCal getTime       In the API I am seeing the below comment but I am not able to understand much of it          calls  cal setTimeZone EST   cal set HOUR  1   cal setTimeZone PST          Is cal set to 1 o clock EST or 1 o clock PST   Answer  PST   More        generally  a call to setTimeZone   affects calls to set   BEFORE AND        AFTER it up to the next call to complete      Could you please help me   One Possible Solution         Calendar cSchedStartCal   Calendar getInstance TimeZone getTimeZone  GMT         long gmtTime   cSchedStartCal getTime   getTime         long timezoneAlteredTime   gmtTime   TimeZone getTimeZone  Asia Calcutta   getRawOffset        Calendar cSchedStartCal1   Calendar getInstance TimeZone getTimeZone  Asia Calcutta         cSchedStartCal1 setTimeInMillis timezoneAlteredTime     Is this solution ok

User · Accepted Answer

In Java  Dates are internally represented in UTC milliseconds since the epoch  so timezones are not taken into account  that s why you get the same results  as getTime   gives you the mentioned milliseconds   In your solution   Calendar cSchedStartCal   Calendar getInstance TimeZone getTimeZone  GMT     long gmtTime   cSchedStartCal getTime   getTime     long timezoneAlteredTime   gmtTime   TimeZone getTimeZone  Asia Calcutta   getRawOffset    Calendar cSchedStartCal1   Calendar getInstance TimeZone getTimeZone  Asia Calcutta     cSchedStartCal1 setTimeInMillis timezoneAlteredTime     you just add the offset from GMT to the specified timezone   Asia Calcutta  in your example  in milliseconds  so this should work fine     Another possible solution would be to utilise the static fields of the Calendar class     instantiates a calendar using the current time in the specified timezone Calendar cSchedStartCal   Calendar getInstance TimeZone getTimeZone  GMT       change the timezone cSchedStartCal setTimeZone TimeZone getTimeZone  Asia Calcutta       get the current hour of the day in the new timezone cSchedStartCal get Calendar HOUR OF DAY     Refer to stackoverflow com questions 7695859  for a more in-depth explanation

User · Answer

The class Date Timestamp represents a specific instant in time  with millisecond precision  since January 1  1970  00 00 00 GMT  So this time difference  from epoch to current time  will be same in all computers across the world with irrespective of Timezone  Date Timestamp doesn t know about the given time is on which timezone   If we want the time based on timezone we should go for the Calendar or SimpleDateFormat classes in java  If you try to print a Date Timestamp object using toString    it will convert and print the time with the default timezone of your machine  So we can say  Date Timestamp  getTime   object will always have UTC  time in milliseconds  To conclude  Date getTime   will give UTC time  but toString   is on locale specific timezone  not UTC    Now how will I create change time on specified timezone   The below code gives you a date  time in milliseconds  with specified timezones  The only problem here is you have to give date in string format      DateFormat dateFormat   new SimpleDateFormat  yyyyMMdd HH mm ss       dateFormatLocal setTimeZone timeZone      java util Date parsedDate   dateFormatLocal parse date     Use dateFormat format for taking input Date  which is always UTC   timezone and return date as String    How to store UTC GMT time in DB   If you print the parsedDate object  the time will be in default timezone  But you can store the UTC time in DB like below           Calendar calGMT   Calendar getInstance TimeZone getTimeZone  GMT             Timestamp tsSchedStartTime   new Timestamp  parsedDate getTime             if  tsSchedStartTime    null                stmt setTimestamp 11  tsSchedStartTime  calGMT              else               stmt setNull 11  java sql Types DATE

[java] How to change TIMEZONE for a java.util.Calendar/Date

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