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From "Duncan Jones (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LANG-796) DateUtils.addDays does not work properly with daylight saving time (DST)
Date Sat, 22 Sep 2012 10:45:07 GMT

    [ https://issues.apache.org/jira/browse/LANG-796?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13461108#comment-13461108
] 

Duncan Jones commented on LANG-796:
-----------------------------------

I agree with Nicola. A class that intends to work with Date values should not be interested
in daylight savings. I suspect this was an unintentional error on the part of the original
implementation.

At the very least, the Javadocs need to change to reflect this behaviour.

The issue with changing the code is that people may already be relying on this behaviour.
All of the {{addXXX}} methods use Calendars and would be impacted.
                
> DateUtils.addDays does not work properly with daylight saving time (DST)
> ------------------------------------------------------------------------
>
>                 Key: LANG-796
>                 URL: https://issues.apache.org/jira/browse/LANG-796
>             Project: Commons Lang
>          Issue Type: Bug
>          Components: lang.time.*
>    Affects Versions: 2.6
>            Reporter: Nicola Barbiero
>
> DateUtils.addDays does not work properly with daylight saving time.
> The signature of the method is 
>       Date addDays(Date date, int amount)
> and the javadocs says "Adds a number of days to a date returning a new object. The original
date object is unchanged",
> so if X=date.getTime() is the number of milliseconds of the date in input,
> the expected behaviour is that the returned Date has a number of milliseconds equal to
X+amount*(86400000), where 86400000 is the number of milliseconds in one day.
> But when the calculation goes across the DST change date, the number of milliseconds
added does not correspond to whole days.
> For example, here in Brussels, this code fragment:
>    Date input = DateUtils.parseDateStrictly("25-03-2012_00:00", new String[] { "dd-MM-yyyy_HH:mm"
});
>    Date output = DateUtils.addDays(input, 1);
> will give:
> 'input' equals to "Sun Mar 25 00:00:00 CET 2012"    ==> input.getTime() equals to
1332630000000
> 'output' equals to "Mon Mar 26 00:00:00 CEST 2012"  ==> output.getTime() equals to
1332712800000
> where 1332712800000-1332630000000=82800000 < 86400000
> (in fact 82800000 is equivalent to 23h).
> Since addDays is working with objects Date, it should not be influenced by events like
the DST.
> Proposed solution: replace the current implementation
> public static Date add(Date date, int calendarField, int amount) {
>         if (date == null) {
>             throw new IllegalArgumentException("The date must not be null");
>         }
>         Calendar c = Calendar.getInstance();
>         c.setTime(date);
>         c.add(calendarField, amount);
>         return c.getTime();
>     }
> based on Calendar with an implementation that works only with Date objects, for example:
> public static Date add(Date date, int calendarField, int amount) {
>         if (date == null) {
>             throw new IllegalArgumentException("The date must not be null");
>         }
>         return new Date(input.getTime() + amount * 86400000l);
>     }

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