A month is not a fixed number of days. Calendar months have either 31, 30, or 28 days (29 in leap years). As there are 12 months in a year, the average is 30.4 days for an average month. Using 30 days as an average can introduce increasingly large errors, but is acceptable for estimation purposes.
The months of the year:
January, March, May, July, August, October, December - 31 days
April, June, September, November - 30 days
February - 28 days, 29 days in leap years (most years evenly divisible by 4)