PREVIOUS_MONTHNAME implementation not robust

https://github.com/jperelli/Redmine-Periodic-Task/blob/fa4ee5ecc96914d4348697e6b507f8cb09fc3e6e/app/models/periodictask.rb#L83C63-L83C70

The PREVIOUS_MONTHNAME calculation simply substracts 30 days.
This leads to wrong resolutions, e.g. when the ticket is created on first of march (then it says "January", obviously)
I suppose, it wont work for other border cases as well (e.g. when run on 31st?)