You don't need to use for-loop to obtain number of days for each month. The following example generates number of days per month from Jan. to Dec. in 2017.
t = datetime(2017,1,1):calmonths(1):datetime(2018,1,1);
daysPerMonth = days(diff(t));
The result is:
>> daysPerMonth
daysPerMonth =
31 28 31 30 31 30 31 31 30 31 30 31