Java server and client are in different timezones: Convert Timestamp / time to ZonedDateTime for comparison

If you are comparing a Timestamp from a remote server located in a different timezone with a local Timestamp, you need to convert either the remote Timestamp to a local one or vice versa.

You need to know your server's timezone id no matter which way you are going to convert. Let's say your server's timezone id is "America/New_York". (You can get all the available timezone ids by calling TimeZone.getAvailableIDs().)

1. Convert a remote Timestamp to a local ZonedDateTime

          String remoteZoneId = "America/New_York";

          TimeZone remoteTimeZone = TimeZone.getTimeZone(serverZoneId);

          Timestamp remoteTimestamp = <the value to be converted>

          //Firstly, create a ZonedDateTime representing the remote Timestamp

          GregorianCalendar remoteCalendar = new GregorianCalendar(remoteTimeZone);

          remoteCalendar.setTimeInMillis(remoteTimestamp.getTime());

          ZonedDateTime remoteZDT = remoteCalendar.toZonedDateTime();

         // ( OR use the following code to get the remote ZonedDateTime:

         //       Instant remoteInstant = Instant.ofEpochMilli(remoteTimestamp.getTime());
         //       ZonedDateTime remoteZDT = remoteInstant.atZone(remoteTimeZone.toZoneId());
         // )

          //Secondly, convert this remote ZonedDateTime to a local ZonedDateTime

          TimeZone localTimeZone = TimeZone.getDefault();

          ZonedDateTime remoteZonedResult;

          if (remoteTimeZone.getRawOffset() == localTimeZone.getRawOffset()){

                   remoteZonedResult = remoteZDT;

          } else {

                  remoteZonedResult = remoteZDT.withZoneSameInstant(localTimeZone.toZoneId());

          }

2. Convert a local Timestamp to a local ZonedDateTime

          Timestamp localTimeStamp = <your local time stamp>

          TimeZone localTimeZone = TimeZone.getDefault();

          ZonedDateTime localZonedResult = ZonedDateTime.of(localTimeStamp.toLocalDateTime(),           localTimeZone.toZoneId());

3. Comparing

           remoteZonedResult.isBefore(localZonedResult);

           remoteZonedResult.isAfter(localZonedResult);

           remoteZoneResult.equals(localZonedResult);

Of course, you can also convert the local Timestamp to a remote ZonedDateTime, and then comparing it with the remote ZonedDateTime representing a remote Timestamp.

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