R1:bdef566f2e28

Fix Excel export of date columns

Summary:
In D4567, I made column formatting more strict, but possibly too strict. @anjali reports date columns showing internal Excel date formats ("42391.2292", etc).

@jack, if you have a chance, can you apply this and verify the behavior with @anjali? Repro steps should be:

- View any tasks in Maniphest.
- Click "Export to Excel".
- Open document in Excel.
- Date column should show dates, not integers around 42,000.

Otherwise I'll test this locally, I just…