perl> '2004-09-01 17:31:45' lt '2009-05-01 19:41:31'
-> 1;
perl> '2010-09-01 17:31:45' gt '2009-05-01 19:41:31'
-> 1;
perl> '2010-09-01' lt '2009-05-01 19:41:31'
-> '';
perl> '2010-09-01' gt '2009-05-01 19:41:31'
-> 1;
perl> '2010-09-01' lt '2009-05-01 19:41:31'
-> '';