SetProperty - add error when setting a date/time offset results in an invalidate date/time

[smalers]

The SetProperty command allows setting a date/time using syntax like CurrentToDay - 1Month. The underlying math apparently does not check for an invalid output in some cases. As an example, if today is March 31 and the result would be February 31, an error is not generated. It may be that use cases typically use an offset of 30 days, for example, which is properly handled. Because an offset of month is somewhat arbitrary because different months have different lengths, the code and tests may have worked fine for common use cases.