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