The natural month refers to the 1st of each month to the last day of the month, and the billing month is the starting date specified by them.

Extension:

Generally there is also an unnatural month:

The unnatural month is relative to the natural month.

Natural month: A natural month starts from the first day of each month and ends on the last day of each month, such as March 1st to March 31st.

Non-natural month: It starts from any day in a month and ends on the same day in the next month. It is also counted as a month, but it is not a natural month. For example, March 15th April 15th is an unnatural month.