TimeZone’s Problem

In my intern project, I need to handle timezone problem. Because in US, there is Daylight time saving and 5 diffferent timezone. My code should provide service for people in different timezones. Therefore, in the front end, I could allow user input time in their own timezone, but I should save date and time in GMT+0. Then I could make sure all time in database is saved in the same timezone. So, it is much easier to perform schedule.

Idea and problem

The idea is quite straight forward.

There is one thing should mentioned. The default timezone of all Azure Virtual machine is GMT+0. You could change that in APPLICATION_SETTING

But here, since user could from different timezone, we did not set this value here.

Java Code implementation

You could find lots of code on StackOverflow telling you how to change timezone, but they all could not use here. The reason is they all assume the input time’s timezone is the same as the local machine time. For example, after parsing datetime from user input, it will use system timezone to understand this time and then you could set timezone id to change it. However, in my case, I want my web application to receive time and parse it to UTC. Because when my RESTful server receive that time, it thought it is GMT+0. Therefore, when I just set it to GMT+0 timezone, it would not change.

// WRONG WAY in THIS CASE!
String dateInString = "22-1-2015 10:15:55 AM";
LocalDateTime ldt = LocalDateTime.parse(dateInString, DateTimeFormatter.ofPattern(DATE_FORMAT));

ZoneId singaporeZoneId = ZoneId.of("Asia/Singapore");
System.out.println("TimeZone : " singaporeZoneId)
//LocalDateTime + ZoneId = ZonedDateTime
ZonedDateTime asiaZonedDateTime = ldt.atZo(singaporeZoneId);
System.out.println("Date (Singapore) : " asiaZonedDateTime)
ZoneId newYokZoneId = ZoneId.("America/New_York");
System.out.println("TimeZone : " + newYokZoneId);

So the right method is to parse time in custom timezone. Then you could just put that in GMT+0 timezone.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy HH:mm:ss");

ZonedDateTime time = LocalDateTime.parse(end, formatter).atZone(ZoneId.of("America/New_York"));
time.format(DateTimeFormatter.ISO_INSTANT).toString());

JavaScript implementation

For Javascript part, we could use Moment package. In my case, I need to change timezone from GMT+0 to user local timezone. It is quite easy, because it is only one line.

moment(UTCTimeString).format('MM/DD/YYYY HH:mm:ss')

Conclusion

The function seems very easy, but I actually spent lots of time on it. The reason is I did not get that the machine’s timezone was “GMT+0” in the beginning, therefore I could not change it.