September has no day 31.

perldoc -f Time::Local

 It is worth drawing particular attention to the expected ranges for t
+he values provided. The value for the day of the
       month is the actual day (ie 1..31), while the month is the numb
+er of months since January (0..11). This is consistent
       with the values returned from "localtime()" and "gmtime()".
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Jan = 0, Feb = 1; Aug = 7; Sept = 8; Dec = 11.