R1:b12f13efd8d0

Force date/time preferences to valid values

Summary:
Fixes T8601. To reproduce the problem:

- Set your time preference to `""` (the empty string). This isn't possible from the modern UI, but can be done with "Right Click > Inspect Element", or users may have carried it forward from an older setting (this is the case with me and @hach-que on this install).
- Load Calendar with some events.
- This parses an epoch, which sets `valueTime` to `""` (since there are no format characters in the preference)…