2.6.0 stopped supporting freezegun for pandas dataframe to_excel export

Created originally on Bitbucket by georgepicsa (George van Rooyen)

I've been using the package freezegun (https://github.com/spulec/freezegun) for testing, and it works up to the latest 2.5 version of openpyxl, but it's failing for 2.6.0. It looks like there might have been a change in the is_date_format function that caused this.

The screenshot below is from a test that uses the freeze_time decorator before the test, like so:

@freeze_time("2016-10-30 12:00:01")
def test_...