timedelta bug fix
(was initially the same as !389 (closed), but rebased on the 3.0 branch, and now dropped one commit)
This MR changes two minor details in the code for reading time/date values from xlsx:
- 0 is turned into time(0,0) instead of a full datetime
- Negative serial dates are always turned into full datetimes, never time only.
Both changes are in line with the xlsx specification. This MR makes it possible to reliably retrieve timedelta values from xlsx files using the 1904 date system, including negative timedeltas.