2.6.0 stopped supporting freezegun for pandas dataframe to_excel export
*Created originally on Bitbucket by [georgepicsa (George van Rooyen)](https://bitbucket.org/%7B43de307e-ba63-4612-a8b1-508f6b61ddf4%7D/)* 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_... ``` ![openpyxl stacktrace.png](https://bitbucket.org/repo/RMdExx/images/1600807379-openpyxl%20stacktrace.png)
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