We are using openpyxl to create some reports which we are testing using freezegun. Freezegun is a popular library to test. I don't think it is much code and is the same as for pandas. It would also fix #1179 (closed).
I added a FREEZEGUN constant just like PANDAS in compat/numbers.py, but instead of using that in cells.py I followed example of pandas and just tried to import again.