Monthly Data (Freq = 720) = 30 days, but which month has 30 days.

[endisclu]

Since no month has 30 days, I'm wondering if the window will be postponed every time. For example, I always have a sum on the first day of the month and would like to take this into account. However, given the frequency, I don't have the first of the month in February, for example.

Am I misunderstanding something or do I need to adjust my data?