   STR_TO_DATE(str,format)
This is the reverse function of the DATE_FORMAT() function. It takes a 
string str and a format string format, and returns a 
DATETIME value.
The date, time, or datetime values contained in str should be given 
in the format indicated by format. For the specifiers that can be 
used in format, see the table in the DATE_FORMAT() function 
description. All other characters are just taken verbatim, thus not being 
interpreted.
If str contains an illegal date, time, or datetime value, 
STR_TO_DATE() returns NULL.
