Time::Local let me faint

Hello,lists,

Please see these two lines' output:

[$ perl -Mstrict -MTime::Local -le 'print timelocal(0,0,0,31,8,2006)'
Day '31' out of range 1..30 at -e line 1

$ perl -Mstrict -MTime::Local -le 'print timelocal(0,0,0,31,7,2006)'
1156953600


I translate the time of '2006-7-31 00:00:00' to unix timestamp,it's
successful.
But when I translate the time of '2006-8-31 00:00:00' to unix timestamp,it
said '31 out of range'.
I'm so faint that August doesn't have 31th day?Please tell me why this
happen and how to resolve it.
Thank you very much.