Work around MySQL bug #62077

The use of DATE_SUB can, depending on the MySQL table's colocation, cause errors like this: "Illegal mix of collations (utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,NUMERIC) for operation '<='". I think this is probably related to MySQL bug #62077 - http://bugs.mysql.com/bug.php?id=62077

In this patch, I've worked around it by doing the date arithmetic in PHP instead. The fix works for me (in my copy of the semaphore library in http://wordpress.org/plugins/updraftplus - I had a user who hit this problem).