Java: Convert MST to EST

Question

I've been trying to convert the time since epoch until today and display it in Eastern Standard Time. Here is what outputs on the remote machine (it's remotely hosted):

Date now = new Date(System.currentTimeMillis());
System.out.println(now.toString());
// Thu Apr 24 14:36:11 MST 2014

No idea what MST is, but I want to get the current milliseconds since epoch in EST, and display the result in EST.

No matter what I do, I can't get daylights savings to work (it's currently Daylights Savings Time in the EST Time Zone); I either end up in PST, GMT or UTC, and when I do get "EST" it's either some random value or 1 hour behind or 3 hours behind.

I would like the output to be formatted using this DateFormat:

DateFormat EXPIRE_FORMAT = new SimpleDateFormat("MMM dd, yyyy h:mm a z");

Answer 1

Just set the time zone you want the time to be displayed in using DateFormat#setTimeZone(TimeZone)

Date now = new Date(System.currentTimeMillis());
DateFormat EXPIRE_FORMAT = new SimpleDateFormat("MMM dd, yyyy h:mm a z");
EXPIRE_FORMAT.setTimeZone(TimeZone.getTimeZone("America/Montreal")); // or whatever relevant TimeZone id 
System.out.println(EXPIRE_FORMAT.format(now));

AFAIK, there is no EST currently. It's all EDT in Spring.

The above prints

Apr 24, 2014 5:53 PM EDT

Answer 2

The comments and the answer by Sotirios Delimanolis are correct.

Avoid 3 or 4 Letter Time Zone Codes

You should avoid the 3 or 4 letter codes for time zones as they are neither standardized nor unique. Instead use proper time zone names, usually a continent+city.

Avoid j.u.Date

The java.util.Date and .Calendar & SimpleDateFormat classes bundled with Java are notoriously troublesome. Use a decent date-time library with an updated time zone database. For Java, that means either Joda-Time or the new java.time package in Java 8 (inspired by Joda-Time).

Avoid Milliseconds-Since-Epoch

I suggest you avoid working with milliseconds since epoch. Gets confusing fast as the number is meaningless when read by a human. Let the date-time library manage the milliseconds for you.

Specify Time Zone

Generally best to specify the desired/intended time zone. If you omit the time zone, all the major date-time libraries (java.util.Date, Joda-Time, java.time) apply the JVM's default time zone.

Joda-Time Example

Example code in Joda-Time 2.3.

DateTimeZone timeZoneToronto = DateTimeZone.forID( "America/Toronto" );
DateTime dateTimeToronto = new DateTime( timeZoneToronto ); // Current moment.
DateTime dateTimeUTC = dateTimeToronto.withZone( DateTimeZone.UTC );
DateTime dateTimeParis = dateTimeToronto.withZone( DateTimeZone.forID( "Europe/Paris" ) );

If you really want the milliseconds since epoch, call the getMillis method. In example code above, all three DateTime objects have the same number of milliseconds-since-epoch.

long millis = dateTimeToronto.getMillis();

If you need a java.util.Date for use with other classes…

java.util.Date date = dateTimeToronto.toDate();

While Joda-Time uses the ISO 8601 standard formats as its defaults, you may specify other formats for generating strings.

DateTimeFormatter formatter = DateTimeFormat.forPattern( "MMM dd, yyyy h:mm a z" );
String output = formatter.print( dateTimeToronto );