Converting between java time LocalDateTime and java util Date

Question

Java 8 has a completely new API for date and time  One of the most useful classes in this API is LocalDateTime  for holding a timezone-independent date-with-time value    There are probably millions of lines of code using the legacy class java util Date for this purpose  As such  when interfacing old and new code there will be a need for converting between the two  As there seems to be no direct methods for accomplishing this  how can it be done

User · Answer

I m not sure if this is the simplest or best way  or if there are any pitfalls  but it works   static public LocalDateTime toLdt Date date        GregorianCalendar cal   new GregorianCalendar        cal setTime date       ZonedDateTime zdt   cal toZonedDateTime        return zdt toLocalDateTime       static public Date fromLdt LocalDateTime ldt        ZonedDateTime zdt   ZonedDateTime of ldt  ZoneId systemDefault         GregorianCalendar cal   GregorianCalendar from zdt       return cal getTime

User · Answer

Much more convenient way if you are sure you need a default timezone    Date d   java sql Timestamp valueOf  myLocalDateTime

User · Answer

The fastest way for LocalDateTime -  Date is   Date from ldt toInstant ZoneOffset UTC

User · Answer

the following seems to work when converting from new API LocalDateTime into java util date   Date from ZonedDateTime of  time as LocalDateTime   ZoneId systemDefault    toInstant       the reverse conversion can be  hopefully  achieved similar way     hope it helps

User · Answer

Short answer   Date in   new Date    LocalDateTime ldt   LocalDateTime ofInstant in toInstant    ZoneId systemDefault     Date out   Date from ldt atZone ZoneId systemDefault    toInstant       Explanation   based on this question about LocalDate   Despite its name  java util Date represents an instant on the time-line  not a  date   The actual data stored within the object is a long count of milliseconds since 1970-01-01T00 00Z  midnight at the start of 1970 GMT UTC    The equivalent class to java util Date in JSR-310 is Instant  thus there are convenient methods to provide the conversion to and fro   Date input   new Date    Instant instant   input toInstant    Date output   Date from instant     A java util Date instance has no concept of time-zone  This might seem strange if you call toString   on a java util Date  because the toString is relative to a time-zone  However that method actually uses Java s default time-zone on the fly to provide the string  The time-zone is not part of the actual state of java util Date   An Instant also does not contain any information about the time-zone  Thus  to convert from an Instant to a local date-time it is necessary to specify a time-zone  This might be the default zone - ZoneId systemDefault   - or it might be a time-zone that your application controls  such as a time-zone from user preferences  LocalDateTime has a convenient factory method that takes both the instant and time-zone   Date in   new Date    LocalDateTime ldt   LocalDateTime ofInstant in toInstant    ZoneId systemDefault       In reverse  the LocalDateTime the time-zone is specified by calling the atZone ZoneId  method  The ZonedDateTime can then be converted directly to an Instant   LocalDateTime ldt       ZonedDateTime zdt   ldt atZone ZoneId systemDefault     Date output   Date from zdt toInstant       Note that the conversion from LocalDateTime to ZonedDateTime has the potential to introduce unexpected behaviour  This is because not every local date-time exists due to Daylight Saving Time  In autumn fall  there is an overlap in the local time-line where the same local date-time occurs twice  In spring  there is a gap  where an hour disappears  See the Javadoc of atZone ZoneId  for more the definition of what the conversion will do   Summary  if you round-trip a java util Date to a LocalDateTime and back to a java util Date you may end up with a different instant due to Daylight Saving Time   Additional info  There is another difference that will affect very old dates  java util Date uses a calendar that changes at October 15  1582  with dates before that using the Julian calendar instead of the Gregorian one  By contrast  java time   uses the ISO calendar system  equivalent to the Gregorian  for all time  In most use cases  the ISO calendar system is what you want  but you may see odd effects when comparing dates before year 1582

User · Answer

Here is what I came up with   and like all Date Time conundrums it is probably going to be disproved based on some weird timezone-leapyear-daylight adjustment  D    Round-tripping   Date  lt  lt -   LocalDateTime  Given  Date date    some date    1  LocalDateTime  lt  lt  Instant lt  lt  Date      Instant instant   Instant ofEpochMilli date getTime         LocalDateTime ldt   LocalDateTime ofInstant instant  ZoneOffset UTC      2   Date  lt  lt  Instant  lt  lt  LocalDateTime      Instant instant   ldt toInstant ZoneOffset UTC       Date date   Date from instant     Example   Given   Date date   new Date    System out println date     long      date getTime        1  LocalDateTime  lt  lt  Instant lt  lt  Date   Create Instant from Date   Instant instant   Instant ofEpochMilli date getTime     System out println  Instant from Date  n    instant     Create Date from Instant  not necessary but for illustration    date   Date from instant   System out println  Date from Instant  n    date     long      date getTime       Create LocalDateTime from Instant  LocalDateTime ldt   LocalDateTime ofInstant instant  ZoneOffset UTC   System out println  LocalDateTime from Instant  n    ldt      2  Date  lt  lt  Instant  lt  lt  LocalDateTime  Create Instant from LocalDateTime   instant   ldt toInstant ZoneOffset UTC   System out println  Instant from LocalDateTime  n    instant     Create Date from Instant   date   Date from instant   System out println  Date from Instant  n    date     long      date getTime       The output is   Fri Nov 01 07 13 04 PDT 2013 long  1383315184574  Instant from Date  2013-11-01T14 13 04 574Z  Date from Instant  Fri Nov 01 07 13 04 PDT 2013 long  1383315184574  LocalDateTime from Instant  2013-11-01T14 13 04 574  Instant from LocalDateTime  2013-11-01T14 13 04 574Z  Date from Instant  Fri Nov 01 07 13 04 PDT 2013 long  1383315184574

User · Answer

Everything is here   http   blog progs be 542 date-to-java-time  The answer with  round-tripping  is not exact   when you do   LocalDateTime ldt   LocalDateTime ofInstant instant  ZoneOffset UTC     if your system timezone is not UTC GMT  you change the time

User · Answer

I think below approach will solve the conversion without taking time-zone into consideration  Please comment if it has any pitfalls      LocalDateTime datetime   input     public static final DateTimeFormatter yyyyMMddHHmmss DATE FORMAT   DateTimeFormatter ofPattern  quot yyyy-MM-dd HH mm ss quot        String formatDateTime   datetime format yyyyMMddHHmmss DATE FORMAT       Date outputDate   new SimpleDateFormat  quot yyyy-MM-dd HH mm ss quot   parse formatDateTime     output

User · Answer

If you are on android and using threetenbp you can use DateTimeUtils instead   ex     Date date   DateTimeUtils toDate localDateTime atZone ZoneId systemDefault    toInstant       you can t use Date from since it s only supported on api 26

[java] Converting between java.time.LocalDateTime and java.util.Date

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