I seems found the rule of those issues :
1) tim74's issue :
The error is: events dates is recalculated every month. On single number (1,3,5,7,9,11) months, the dates is correct.
But on double number (2,4,6,8,10,12) months, the dates is incorrect.
(I even change system-time to test)
If create 5 events, like following images show,
you can see, at 8/1 (m/d), only the 1st event is correct.
2) my issue :
Use the above same issues.
And, add a setting :
Options >> Calendars >> Default >> Default alarm (minutes) >> 0
then, you can see that the alarm window will display strange strings for next trigger time.