There is a bug in Kernel/Modules/AgentStats.pm which does not accept absolute dates when creating or editing reports/statistics:
[Error][Kernel::System::Time::TimeStamp2SystemTime][Line:208]: 2013-07-00 11:23:59
--- Kernel/Modules/AgentStats.pm.orig 2013-03-05 20:43:50.000000000 +1300
+++ Kernel/Modules/AgentStats.pm 2013-07-11 09:27:37.522545270 +1200
@@ -953,7 +953,6 @@
"%04d-%02d-%02d %02d:%02d:%02d",
$Time{ $Limit . 'Year' },
$Time{ $Limit . 'Month' },
- $Time{ $Limit . 'Week' },
$Time{ $Limit . 'Day' },
$Time{ $Limit . 'Hour' },
$Time{ $Limit . 'Minute' },