Equality operators are defined for wxDateSpans. Two datespans are equal if
and only if they both give the same target date when added to {\bf every}
source date. Thus wxDateSpan::Months(1) is not equal to wxDateSpan::Days(30),
-because they not give the same date when added to 1 Feb. But
+because they don't give the same date when added to 1 Feb. But
wxDateSpan::Days(14) is equal to wxDateSpan::Weeks(2)
Finally, notice that for adding hours, minutes and so on you don't need this