When adding a month to the date, all lesser components (days, hours, ...)
won't be changed unless the resulting date would be invalid: for example,
-Jan 31 + 1 month will be Feb 28, not (non existing) Feb 31.
+Jan 31 + 1 month will be Feb 28, not (non-existing) Feb 31.
Because of this feature, adding and subtracting back again the same
wxDateSpan will {\bf not}, in general give back the original date: Feb 28 - 1
function.
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}
+and only if they both give the same target date when added to {\bf every}\rtfsp
source date. Thus wxDateSpan::Months(1) is not equal to wxDateSpan::Days(30),
because they don't give the same date when added to 1 Feb. But
wxDateSpan::Days(14) is equal to wxDateSpan::Weeks(2)