Joda-Time is a standard date and time library for Java.

http://www.joda.org/joda-time/

Issue 77: https://github.com/JodaOrg/joda-time/issues/77

Problem:
after calling addDays(0), value of MutableDateTime while it shouldn't

Test case:

final MutableDateTime mdt = new MutableDateTime(2011, 10, 30, 3, 0, 0, 0, DateTimeZone.forID("Europe/Berlin"));
System.out.println("Start date:   " + mdt + " (" + mdt.toInstant().getMillis() + ")");
mdt.addHours(-1);
System.out.println("addHours(-1): " + mdt + " (" + mdt.toInstant().getMillis() + ")");
mdt.addHours(0);
System.out.println("addHours(0):  " + mdt + " (" + mdt.toInstant().getMillis() + ")");
mdt.addDays(0);
System.out.println("addDays(0):   " + mdt + " (" + mdt.toInstant().getMillis() + ")");

Start date:   2011-10-30T03:00:00.000+01:00 (1319940000000)   
addHours(-1): 2011-10-30T02:00:00.000+01:00 (1319936400000)  
addHours(0):  2011-10-30T02:00:00.000+01:00 (1319936400000)    
addDays(0):   2011-10-30T02:00:00.000+01:00 (1319932800000)   

Start date:   2011-10-30T03:00:00.000+01:00 (1319940000000)    OK
addHours(-1): 2011-10-30T02:00:00.000+01:00 (1319936400000)    OK
addHours(0):  2011-10-30T02:00:00.000+01:00 (1319936400000)    OK, no change in time
addDays(0):   2011-10-30T02:00:00.000+02:00 (1319932800000)    error, time has changed by 1 hour

Result from Ziyuan:


The best predicate:

hours >= -0.0 (org.joda.time.MutableDateTime:804)

Ziyuan's analysis result is interesting for this one. The original bug report believed that the bug happens when method AddDays is invoked with parameter 0. The fix is as follows.



That is, the authors simply added a check so that when the parameter is 0, nothing is done. The fix obviously made the test case pass. However, what Ziyuan's output means is that the bug is actually not due to the parameter 0. Rather it is due to this parameter -1 used when invoking method AddHours. Our investigation shows that indeed, the bug goes away if we change -1 to 1 without using the bug fix. This shows that in fact the bug is not "fixed" and the bug has been explained wrongly. Subsequently, we made a bug report based on the finding at https://github.com/JodaOrg/joda-time/issues/310. The author of project then confirmed that in fact, that particular time is around a daylight savings change and therefore the result of the test case is in fact correct!