CODEWITHSUNDEEP
javadatedatetime
sandwwraith
sandwwraith

Reputation: 319

How to ignore timezone identifier in date parsing (Java)?

I'm trying to parse date like this:

DateFormat df = new SimpleDateFormat("MMM dd, yyyy K:mm:ss,SSS a z", Locale.ENGLISH);
Date date = df.parse("Oct 04, 2015 2:11:58,757 AM UTC");

And I'm getting a value of 5 hours am, because i live in UTC+3 timezone. But i need to have value of 2am, however, with the same format string(date string given in a specified format, which i'm not allowed to change). How to do this?

Upd: I don't need to format the date in proper timezone, i need to COMPARE these dates by its values without timezones. I want exactly that date have parsed ignoring the timezone in original string - and be always in the same timezone (my, for example), no matter what contains original string: UTC or UTC+3 or something else.

Upvotes: 5

Views: 26519

Answers (2)

Basil Bourque
Basil Bourque

Reputation: 339787

The accepted Answer is working too hard. Manipulating offsets is the province of a date-time library. Doing such work yourself is a waste of your time, and likely to be a source of bugs.

The old java.util.Date/.Calendar classes are notoriously troublesome. Avoid them. Instead use either java.time or Joda-Time.

java.time

Java 8 and later has a new java.time framework built-in.

Confused Question

Your Question is confused. You say you want to ignore time zone, yet you accept an answer that does indeed parse and process the time zone. And that answer then adjusts the result by an offset. So, it seems that you do not want to ignore the time zone.

Indeed, ignoring the time zone rarely makes sense. Perhaps you want to compare a pair of factories in Berlin and in Detroit to see if they both take a lunch break at the same time. In this case you are comparing their respective wall-clock time. The java.time framework offers the “Local” classes for this purpose: LocalDate, LocalTime, and LocalDateTime. But this is rarely needed in most business scenarios in my experience. These objects are not tied to the timeline.

So it seems that what you do want is to be able to compare date-time values across various time zones. The java.time classes do that implicitly. ZonedDateTime objects with various assigned time zones can be compared to one another with isBefore, isAfter, and isEqual methods.

Example Code

First we parse the input string.

The z pattern code means to expect and parse a time zone. The resulting date-time object will also be assigned this object if no other specific time zone is specified.

We also assign a Locale object with a human language component matching the text we expect to see in the input string. In this case, we need any Locale with English.

String input = "Oct 04, 2015 2:11:58,757 AM UTC";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern( "MMM dd, yyyy K:mm:ss,SSS a z" ).withLocale( Locale.ENGLISH );

ZonedDateTime then = ZonedDateTime.parse( input, formatter );

Next we get the current time for Québec. This arbitrary choice of time zone will demonstrate further below that we can compare this ZonedDateTime object to another with a different time zone. Specifically, comparing against the UTC time zone assigned to our then object above.

ZoneId zoneId = ZoneId.of( "America/Montreal" );
ZonedDateTime now = ZonedDateTime.now( zoneId );

Do the comparison.

Boolean isThenBeforeNow = then.isBefore( now );

By the way, generally-speaking, the best practice in date-time work is to convert all your date-time values to UTC time zone for business logic, storage, and data exchange. Adjust into a time zone only as need be to satisfy a user’s expectations on-screen or in reports.

ZonedDateTime nowUtc = now.withZoneSameInstant( ZoneOffset.UTC );

Dump to console.

System.out.println( "input: " + input );
System.out.println( "then: " + then );
System.out.println( "now: " + now );
System.out.println( "isThenBeforeNow: " + isThenBeforeNow );
System.out.println( "nowUtc: " + nowUtc );

When run.

input: Oct 04, 2015 2:11:58,757 AM UTC

then: 2015-10-04T02:11:58.757Z[UTC]

now: 2015-10-19T19:28:04.619-04:00[America/Montreal]

isThenBeforeNow: true

nowUtc: 2015-10-19T23:28:04.619Z

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Upvotes: 12

sandwwraith
sandwwraith

Reputation: 319

Upd2: Solved

Okay, now i get what i want:

DateFormat df = new SimpleDateFormat("MMM dd, yyyy K:mm:ss,SSS a z", Locale.ENGLISH);
Date date = df.parse("Oct 04, 2015 2:11:58,757 AM UTC");
long diff = TimeZone.getDefault().getRawOffset() - df.getTimeZone().getRawOffset();
date = new Date(date.getTime()-diff);

Anyway, thanks for everyone

Upvotes: 2

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