api - Java: How do you convert a UTC timestamp to local time?

Question

I have a timestamp that's in UTC and I want to convert it to local time without using an API call like TimeZone.getTimeZone("PST"). How exactly are you supposed to do this? I've been using the following code without much success:

private static final SimpleDateFormat mSegmentStartTimeFormatter = new        SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");

Calendar calendar = Calendar.getInstance();

    try {
        calendar.setTime(mSegmentStartTimeFormatter.parse(startTime));
    }
    catch (ParseException e) {
        e.printStackTrace();
    }

    return calendar.getTimeInMillis();

Sample input value: [2012-08-15T22:56:02.038Z]

should return the equivalent of [2012-08-15T15:56:02.038Z]

Answer 1

Date has no timezone and internally stores in UTC. Only when a date is formatted is the timezone correction applies. When using a DateFormat, it defaults to the timezone of the JVM it's running in. Use setTimeZone to change it as necessary.

DateFormat utcFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");
utcFormat.setTimeZone(TimeZone.getTimeZone("UTC"));

Date date = utcFormat.parse("2012-08-15T22:56:02.038Z");

DateFormat pstFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");
pstFormat.setTimeZone(TimeZone.getTimeZone("PST"));

System.out.println(pstFormat.format(date));

This prints 2012-08-15T15:56:02.038

Note that I left out the 'Z' in the PST format as it indicates UTC. If you just went with Z then the output would be 2012-08-15T15:56:02.038-0700