Java SimpleDateFormat timezone parsing

Question

I'm curious how Java's SimpleDateFormat decides when to increment/decrement the passed in time based on the timezone it's set to.

Let's say I have a date 06/04/2013. I then set the timezone to one far away from me(I'm in GMT-5). Let's say I use GMT+8.

I call

SimpleDateFormat df = new SimpleDateFormat( "M/d/yy" );
df.setTimeZone( TimeZone.getTimeZone( "GMT+8" ) );
df.parse( endDate ) // this returns 06/**03**/2013  //endDate is just a String

It returns 06/03/2013. Why does it decrement it?

Edit: Basically I'm asking what is the reference point that Java uses to knock back my date to 6/3 if I set it to GMT+8. There's some logic that says hey I'm not on this current timezone so let's change it. But since I'm passing in a String I don't see where it could be.

I assume by default if I don't provide a Timezone in the string it will default to GMT.

JB Nizet · Accepted Answer · 2013-07-10 21:45:28Z

2

You're in GMT-5, and you're parsing a string representing a moment in the GMT+8 time zone.

So, the date 06/04/2013 is in fact 06/04/2013 00:00 GMT+8. To get the date at GMT, you have to subtract 8 hours: 06/03/2013 16:00 GMT. And to get the date at GMT-5, you have to subtract another 5 hours: 06/03/2013 11:00 GMT-5.

All these strings are different representations of the same moment.

edited Jul 10, 2013 at 21:45

answered Jul 10, 2013 at 21:31

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

jdero Over a year ago

I thought he tried GMT-5 and it showed as the 4th, and then he tried GMT+8 and it showed up as the 3rd.

stan Over a year ago

So you're saying that by default SimpleDateFormat assumes I want the date in GMT and not the timezone I pass in?

JB Nizet Over a year ago

No. I precisely said that by setting the timezone of the date format to GMT+8, you're making it parse 06/04/2013 as 06/04/2013 GMT+8, which is the same moment as 06/03/2013 16:00 GMT, which is the same moment as 06/03/2013 11:00 GMT-5

Collectives™ on Stack Overflow

Java SimpleDateFormat timezone parsing

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related