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 need to rebuild some dependencies first which is a bit involved.

Test Plan: None yet.

Reviewers: jack, btrahan

Reviewed By: btrahan

CC: anjali, aran

Differential Revision: https://secure.phabricator.com/D5467