A "GetMonthLastDay()" function that receives as input a specific date and retrieves as output the last day of the month provided (i.e. 28, 29, 30, 31). 

Something similar to this:
This can be very helpful when handling custom date widgets with no day to choose and you still want to filter the range from day 1 of a month to the last day of some future month.

Also useful when you want to know if some date belongs to a leap year (or not).