What is the difference between Monthly and Rolling 30 days granularity in Revenuestory?

The Monthly mode of granularity refers to any time period from the beginning of a month to the end of that month, whereas the Rolling 30 days mode refers to any date from the current month to the same date in the next month.

For Example:

Month: January 1st to January 31st

Rolling 30 days: January 17th to February 17th