LP#1635737 Due date calculation honors variable durations

LP#1635737 Due date calculation honors variable durations

Due date calculations now take time changes, variable length months, and
variable length (leap) years into account.

For example, a duration of "1 month" results in a due date with the same
day the following month, instead of simply adding 365/12=~30 days to
today.

Note, in cases where a matching day in the following month (or year)
does not exist (e.g. Jan 31 + "1 month"), days are added to the result
to accommodate the extra days.  Jan 31 + "1 month" yields March 3rd in
a non-leap year.

Signed-off-by: Bill Erickson <berickxx@gmail.com>