Add DurationTersity.month

[Keithcat1]

Because hearing that a file was modified 32 weeks ago feels kind of unintuitive.

[tejainece]

I did not include month because month does not have fixed number of days. Do you have any suggestions on how to do this elegantly?

It is easy to add Years tho.

[Aqua-Ye]

I think you can just use 30 days per month as a general rule, it doesn't have to be exactly accurate IMO.