My application keeps all Java Date's in UTC. Parsing them is easy, but when I print them out, the display shows the computer's local time zone, and I want to show it as UTC.

Example code:

String sample = "271210 200157 UTC";
SimpleDateFormat dfmt = new SimpleDateFormat("ddMMyy HHmmss Z");
Date result = dfmt.parse(sample);
System.out.printf("%tc\n", result);

the result is

Mon Dec 27 15:01:57 EST 2010

What I want is

开发者_高级运维Mon Dec 27 20:01:57 UTC 2010

Clearly I have to set some Locale and TimeZone values, but I don't see where to put them. Thanks Pat

You can set the time zone for the DateFormat:

  dfmt.setTimeZone(timeZone);

Also, you do not want to print the Date object itself, but again use the DateFormat

  String formattedDate = dfmt.format(result);

Avoid java.util.Date

The old java.util.Date, .Calendar, and SimpleDateFormat classes are notoriously troublesome. Avoid them.

Among the many problems is the fact that a java.util.Date object:

• Has a time zone buried deep inside
• Ignores that time zone for most practical purposes
• Has a toString method implementation that applies the JVM’s current default time zone when generating the String representation of its internal UTC date-time value.

So to a naïve programmer, the Date object seems to have a time zone but is actually always is UTC (just a 64-bit integer inside, a count from epoch).

Bad Format

The format of your input string is clumsy. In particular, using two digits for the year is problematic.

If you have any control, I strongly suggest changing it. The first choice is to follow the standard ISO 8601 formats.

Joda-Time & java.time

Use either Joda-Time or the new java.time package built into Java 8 (and inspired by Joda-Time).

Unlike a java.util.Date, a DateTime object in Joda-Time clearly understands its own assigned time zone. If you omit the time zone, the JVM’s current default time zone is assigned. I recommend always specifying the desired time zone.

Both Joda-Time and java.time use ISO 8601 formats by default for both generating and parsing String representations of date-time values.

In Joda-Time 2.7…

String input = "271210 200157 UTC";
DateTimeFormatter formatter = DateTimeFormat.forPattern( "ddMMyy HHmmss z" ).withZoneUTC();
DateTime dateTimeUtc = formatter.parseDateTime( input );

Easy to adjust time zone. For fun, let's move that UTC value into Québec time zone.

DateTime dateTimeMontréal = dateTimeUtc.withZone( DateTimeZone.forID( "America/Montreal" ) );
String output = DateTimeFormat.forStyle( "FF" ).withLocale( Locale.CANADA_FRENCH ).print( dateTimeMontréal );

Dump to console.

System.out.println( "input : " + input );
System.out.println( "dateTimeUtc : " + dateTimeUtc );
System.out.println( "dateTimeMontréal : " + dateTimeMontréal );
System.out.println( "output : " + output );

input : 271210 200157 UTC
dateTimeUtc : 2010-12-27T20:01:57.000Z
dateTimeMontréal : 2010-12-27T15:01:57.000-05:00
output : lundi 27 décembre 2010 15 h 01 EST