Performing date and time calculations in Java

Question

I'm trying to do simple date/time calculations and no matter what I do I'm getting confused with what appears to be time zones etc.

I need to add (and subtract)different date/times together so I figured the easiest way would be to convert them to seconds, perform the calculations and then format back to a string. But no matter what I do, I'm one hour out - which sounds like timezone/DST etc.

What's wrong with this:

 long dateInMillis = 3600000L;

 DateFormat formatter = new SimpleDateFormat("HH:mm");

 Date dt = new Date();
 dt.setTime(dateInMillis);
 System.out.println( formatter.format(dt.getTime()));

The answer I get is 2:00. Even if I use:

 long dateInMillis = 1;

I still get 1:00

Help please ;-)

TIA Martin

save yourself a lot of headache, use an existing library for this - I like JodaTime myself. — Nim
– Nim, Commented Mar 1, 2012 at 13:48
@Nim - Why not post that as an answer (maybe with some sample code) so we can give you some upvotes! — David Webb
– David Webb, Commented Mar 1, 2012 at 13:51
I was actually going to post this as an answer myself, but I didn't want to steal @Nim's thunder. — Erick Robertson
– Erick Robertson, Commented Mar 1, 2012 at 13:51
@Dave/Erick, I figured others would chime in with Joda anyway - I've added an answer, will update with some examples once the OP's intentions are clearer... — Nim
– Nim, Commented Mar 1, 2012 at 13:57

Mark McLaren · Accepted Answer · 2012-03-01 13:58:42Z

2

I agree that using Joda is likely to make life easier. However what you are seeing is the effect of a TimeZone sensitive formatter.

You could try adding:

formatter.setTimeZone(TimeZone.getTimeZone("UTC"));

answered Mar 1, 2012 at 13:58

Mark McLaren

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Martin Walke Over a year ago

That's the one!!!!!! Thanks Mark. Brilliant!! That works. I would agree with everyone else that Joda is probably better but in my case - can't be used.

Nim · Accepted Answer · 2012-03-01 13:56:29Z

2

Save yourself a lot of headache, use an existing library for this - I like Joda Time myself.

For example the DateTime class supports various calculations (to add or subtract specific periods etc.) If you want an example, post your exact requirement, and I'll add it to the answer.

answered Mar 1, 2012 at 13:56

Nim

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Martin Walke Over a year ago

Thanks all for your responses.Ok - should have said at the beginning. I have to use the basic libraries as I'm unable to load anything else on the target machine.

tveon · Accepted Answer · 2012-03-01 13:53:04Z

0

You should not try and format a long as a date. Just use:

long dateInMillis = 3600000L;

DateFormat formatter = new SimpleDateFormat("HH:mm");

Date dt = new Date();
dt.setTime(dateInMillis);
System.out.println( formatter.format(dt));

However, I would use a 3rd party lib for handling time-calculations like JodaTime as @Nim suggested, if your calculations are in anyway complicated.

answered Mar 1, 2012 at 13:53

tveon

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Martin Walke Over a year ago

OK - but that still gives me 2:00

Stefan · Accepted Answer · 2012-03-01 13:57:34Z

0

I also suggest to use a framework as suggested in the comments.

However, Date is based on the milliseconds after Jan. 1st 1970. So 3600000L is just one hour from there, so 1:00 is correct.

answered Mar 1, 2012 at 13:57

Stefan

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Martin Walke Over a year ago

That's just the problem. I don't 1:00, I get 2:00. BUt Mark's solution wrioks.

Luiggi Mendoza · Accepted Answer · 2012-03-01 14:04:53Z

0

According to people here, you could use Joda Time to resolve this calculations. If you want to stay with pure Java code, you can look for Calendar class to do your operations with dates. Here is a sample:

Calendar c = Calendar.getInstance();
DateFormat df = new SimpleDateFormat("HH:mm");
Date d = c.getTime();
System.out.println(df.format(d)); //actual hour:minute
c.add(Calendar.HOUR, -4);
d = c.getTime();
System.out.println(df.format(d)); //actual hour:minute minus 4 hours

Still, I won't rely too much in this kind of code. You'll have less problems using Joda Time.

answered Mar 1, 2012 at 14:04

Luiggi Mendoza

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5 Comments

Martin Walke Over a year ago

Thanks Luiggi. I tried the calendar object but get the same results. My date/times are in seconds so can I use :

Luiggi Mendoza Over a year ago

@MartinWalke it could be pretty tedious but you can even add or substract milliseconds on a Calendar instance. It would be better if you post your exact requirement and I'll edit my answer to solve it using Calendar methods.

Martin Walke Over a year ago

Thanks Luiggi. Unfortunately, I only have the time in seconds. I would first have to convert to day/month/year/hr/mins/sec to use the code.addcode method.

Luiggi Mendoza Over a year ago

@MartinWalke you can use Calendar c = Calendar.getInstance(); long l = c.getTimeInMillis() / 1000 so you get the time in seconds, use the value for your math operations and convert it again into a Date using c.setTimeInMillis(l*1000)

Luiggi Mendoza Over a year ago

@MartinWalke I've already tested Mark's answer, and its the right solution for your problem :).

dhblah · Accepted Answer · 2012-03-01 14:13:35Z

0

When you're calling new Date(), current date in current timezone is created. When you're calling 'dt.setTime(dateInMillis);' you set offset in milliseconds from 1 January 1970 in GMT timezone. Let me explain, here in Msk, first call will give me Thu Mar 01 17:02:22 MSK 2012, the second call will give me 1 hour offset, thus in Msk it will be original offset (+3) plus 1, so it'll be Thu Jan 01 04:00:00 MSK 1970. Then when you're calling formatter.format(? extends Number) it calls new Date(((Number)obj).longValue()) inside, so it creates back you date which is Thu Jan 01 04:00:00 MSK 1970 and prints hours and minutes. So, you're right, it prints out time in your current timezone.

answered Mar 1, 2012 at 14:13

dhblah

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Comments

Basil Bourque · Accepted Answer · 2013-12-28 04:20:38Z

For posterity, here's example code using the Joda-Time 2.3 library as mentioned by the other answers and comments.

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.
// import org.joda.time.*;
// import org.joda.time.format.*;

long dateTimeInMillis = 3600000L;  // 1,000 * 60 * 60 = 3600000L = 1 hour.

// Time Zone list… http://joda-time.sourceforge.net/timezones.html  (not quite up-to-date, read page for details)
DateTime dateTimeInUtc = new DateTime( dateTimeInMillis, DateTimeZone.UTC );
// Paris happens to be one hour ahead of UTC/GMT.
DateTime dateTimeInParis = dateTimeInUtc.toDateTime( DateTimeZone.forID( "Europe/Paris" ) );

Dump to console…

System.out.println( "dateTimeInUtc: " + dateTimeInUtc );
System.out.println( "dateTimeInParis: " + dateTimeInParis );

When run…

dateTimeInUtc: 1970-01-01T01:00:00.000Z
dateTimeInParis: 1970-01-01T02:00:00.000+01:00

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