Fix date formatting

[FriedrichFroebel]

This fixes some date formatting issues on some systems as reported in #130. It seems like Windows does not like the - flag for date formatting which is stated as platform-dependent inside the Python docs.

This should close #130.

[tomasbedrich]

This seems to solve the #130. 👍

The alternative solution may be to replace - with # for Windows (as noted on https://strftime.org/). Simple detection could look like this:

import platform
eat_zero_prefix = "#" if platform.system() == "Windows" else "-"
format = {
    "d": r'%{}d'.format(eat_zero_prefix),
    ...
}

Feel free to use whichever solution you like more.

Also, I don't know if it's possible, but we should probably add Windows to test matrix, to prevent this kind of problems.

[FriedrichFroebel]

Oh, must have missed the part regarding #. As this would be the cleaner solution, I am going to replace my current workaround with it.

[FriedrichFroebel]

I currently do not have enough time to investigate CI stuff, but it seems that Windows tests should be available in theory, as in this example.