Datetime updates
This MR has two parts:
- Round datetimes returned by
from_excel()
to milliseconds, which according to the specification is the recommended baseline precision. - Deserialize 1 to 1900-01-01 (instead of 1899-12-31) in the 1900 date system, to avoid jumping dates when moving to 1+ε, and to increase compatibility with Excel.
Edited by nmartensen