Input: string in the format of XX:XX, where X could be either X or a digit (guaranteed).
output: fill the X's so the string will contain the "latest" date on the year in the format of DD:MM, or null if the date is invalid.
for example:
1. the input is "24:XX", the XX will be 12 because the latest date in the year that could fit here is December 24th.
2. the output for "X1:11" the date would be "21:11" beacuse November doesn't have only 30 days and not 31 so "31:11" is invalid
3. the output for "4X:XX" would be null because there is no month with more then 31 days in it.
note: rembember that each month have a different amound of days