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https://issues.apache.org/jira/browse/LANG-796?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13239560#comment-13239560
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Nicola Barbiero commented on LANG-796:
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>From http://docs.oracle.com/javase/6/docs/api/java/util/Date.html:
"The class Date represents a specific instant in time, with millisecond
precision. [...] the Date class is intended to reflect coordinated universal
time (UTC) [...] Nearly all modern operating systems assume that 1 day = 24 ×
60 × 60 = 86400 seconds in all cases."
The concept of daylight saving time (DST) is not present in UTC, and UTC is
NEVER effected by DST, so a method that receives a Date in input and returns a
Date should never be effected by DST.
http://en.wikipedia.org/wiki/Coordinated_Universal_Time#Daylight_saving
The current behavior would be correct if working on Calendar objects, since a
Calendar takes in account the concept of DST.
By the way, even if the final choice will be to not change the current behavior
for this method, at least it should be better documented in its javadoc, to
avoid misuses and misunderstanding in its way of working.
> 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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