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Hello,

I'm a bit confused about Date. In the example below, I get the present
Date, format it to Strings using DateFormaters, reparse the String using
the _same_ DateFormater and recombine them by adding.

The ouput is not the same as the input. Why?

In my locale it outputs:
Before: 23.10.07 19:16:49
After: 23.10.07 18:16:49

Thanks Phil


-- SSCCE --

import java.text.DateFormat;
import java.text.ParseException;
import java.util.Date;

public class TimeTest {
public static void main(String[] args) {
DateFormat df = DateFormat.getDateInstance(DateFormat.SHORT);
DateFormat tf = DateFormat.getTimeInstance(DateFormat.MEDIUM);
DateFormat dtf =
DateFormat.getDateTimeInstance(DateFormat.SHORT,Da teFormat.MEDIUM);

// getting actual time
Date now = new Date();

System.out.println("Before: "+ dtf.format(now));

// separating in date and time strings
String dateString = df.format(now);
String timeString = tf.format(now);

Date date = null;
Date time = null;
try {
// parsing date and time strings
date = df.parse(dateString);
time = tf.parse(timeString);
} catch (ParseException e) {
e.printStackTrace();
}

// recombining date and time
Date after = new Date(date.getTime() + time.getTime());

System.out.println("After: "+ dtf.format(after));
}

}

On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
> The ouput is not the same as the input. Why?
>
> In my locale it outputs:
> Before: 23.10.07 19:16:49
> After: 23.10.07 18:16:49

You forgot about time zones.

Hunter Gratzner wrote:
> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>> The ouput is not the same as the input. Why?
>>
>> In my locale it outputs:
>> Before: 23.10.07 19:16:49
>> After: 23.10.07 18:16:49
>
> You forgot about time zones.
>

Yes, but why is the Date to String conversion time-zone insensitive
while the String to Date is time-zone sensitive?

Philipp wrote On 10/23/07 16:28,:
> Hunter Gratzner wrote:
>
>>On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>
>>>The ouput is not the same as the input. Why?
>>>
>>>In my locale it outputs:
>>>Before: 23.10.07 19:16:49
>>>After: 23.10.07 18:16:49
>>
>>You forgot about time zones.
>>
>
>
> Yes, but why is the Date to String conversion time-zone insensitive
> while the String to Date is time-zone sensitive?

It would be instructive to print the two Dates
that you parse from the text strings, before adding
their times together.

--

Philipp wrote:
> Hunter Gratzner wrote:
>> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>> The ouput is not the same as the input. Why?
>>>
>>> In my locale it outputs:
>>> Before: 23.10.07 19:16:49
>>> After: 23.10.07 18:16:49
>>
>> You forgot about time zones.
>
> Yes, but why is the Date to String conversion time-zone insensitive
> while the String to Date is time-zone sensitive?

Both are time zone sensitive.

Which is exactly why you can not add as you do.

(local date - timezone offset) + (local time - timezone offset)
!=
(local date + local time - timezone offset)

Arne

PS: Unless time zone offset is zero.

Arne Vajhøj wrote:
> Philipp wrote:
>> Hunter Gratzner wrote:
>>> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>>> The ouput is not the same as the input. Why?
>>>>
>>>> In my locale it outputs:
>>>> Before: 23.10.07 19:16:49
>>>> After: 23.10.07 18:16:49
>>>
>>> You forgot about time zones.
>>
>> Yes, but why is the Date to String conversion time-zone insensitive
>> while the String to Date is time-zone sensitive?
>
> Both are time zone sensitive.
>
> Which is exactly why you can not add as you do.
>
> (local date - timezone offset) + (local time - timezone offset)
> !=
> (local date + local time - timezone offset)
>
> Arne
>
> PS: Unless time zone offset is zero.

Right, you should probably use the Calendar class to handle date
manipulation in such a way.


--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

Eric Sosman wrote:
> Philipp wrote On 10/23/07 16:28,:
>> Hunter Gratzner wrote:
>>
>>> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>>
>>>> The ouput is not the same as the input. Why?
>>>>
>>>> In my locale it outputs:
>>>> Before: 23.10.07 19:16:49
>>>> After: 23.10.07 18:16:49
>>> You forgot about time zones.
>>>
>>
>> Yes, but why is the Date to String conversion time-zone insensitive
>> while the String to Date is time-zone sensitive?
>
> It would be instructive to print the two Dates
> that you parse from the text strings, before adding
> their times together.
>

It prints:

Before: 24.10.07 09:03:26
String date: 24.10.07
String time: 09:03:26
Parsed date: 24.10.07 00:00:00
Parsed time: 01.01.70 09:03:26
Added: 24.10.07 08:03:26

Not so instructive IMHO.

Arne Vajhøj wrote:
> Philipp wrote:
>> Hunter Gratzner wrote:
>>> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>>> The ouput is not the same as the input. Why?
>>>>
>>>> In my locale it outputs:
>>>> Before: 23.10.07 19:16:49
>>>> After: 23.10.07 18:16:49
>>>
>>> You forgot about time zones.
>>
>> Yes, but why is the Date to String conversion time-zone insensitive
>> while the String to Date is time-zone sensitive?
>
> Both are time zone sensitive.
>
> Which is exactly why you can not add as you do.
>
> (local date - timezone offset) + (local time - timezone offset)
> !=
> (local date + local time - timezone offset)

Thank you for this clarification! This definitely makes sense.

Although, I'm still not clear how to cleanly solve my problem:
My user inputs a date in one field and a time in another field. I want
to recombine them to one Date object. I can parse each into a Date, but
how should I recombine them? The Calendar class (as suggested by D.
Pitts) does not contain Date arithmetics.

Do I have to break my Date objects into DD.MM.YY HH.MM.SS etc and
reconstruct a Calendar object from this?

Phil

On Wed, 24 Oct 2007 09:25:50 +0200, Philipp wrote:
> Although, I'm still not clear how to cleanly solve my problem:
> My user inputs a date in one field and a time in another field. I want
> to recombine them to one Date object. I can parse each into a Date, but
> how should I recombine them? The Calendar class (as suggested by D.
> Pitts) does not contain Date arithmetics.
>
> Do I have to break my Date objects into DD.MM.YY HH.MM.SS etc and
> reconstruct a Calendar object from this?

Set the timezone in your DateFormats to UTC.

/gordon

--

Philipp wrote:
> Arne Vajhøj wrote:
>> Philipp wrote:
>>> Hunter Gratzner wrote:
>>>> On Oct 23, 7:27 pm, Philipp <sicsic...@freesurf.ch> wrote:
>>>>> The ouput is not the same as the input. Why?
>>>>>
>>>>> In my locale it outputs:
>>>>> Before: 23.10.07 19:16:49
>>>>> After: 23.10.07 18:16:49
>>>>
>>>> You forgot about time zones.
>>>
>>> Yes, but why is the Date to String conversion time-zone insensitive
>>> while the String to Date is time-zone sensitive?
>>
>> Both are time zone sensitive.
>>
>> Which is exactly why you can not add as you do.
>>
>> (local date - timezone offset) + (local time - timezone offset)
>> !=
>> (local date + local time - timezone offset)
>
> Thank you for this clarification! This definitely makes sense.
>
> Although, I'm still not clear how to cleanly solve my problem:
> My user inputs a date in one field and a time in another field. I want
> to recombine them to one Date object. I can parse each into a Date, but
> how should I recombine them? The Calendar class (as suggested by D.
> Pitts) does not contain Date arithmetics.
>
> Do I have to break my Date objects into DD.MM.YY HH.MM.SS etc and
> reconstruct a Calendar object from this?

OK sorry about the noise, got my solution.
I can combine both Strings and parse them with one formatter.

Thank you everybody for your help.
Phil