Function name typo and fixed logical statement

There was a mistyped function name and an incorrect if statement, namely:
dates_i_month -> dates_in_month
if null numbers orelse sum - (hd numbers) < 0 -> sum - (hd numbers) <= 0 

Your current code doesn't validate the day in a year correctly. To reproduce this, try:
what_month(31), what_month(59), what_month(60)
After the proposed fix these should work correctly.