Outlook 2007 Delay Delivery problems

I set Outlook 2007 Delay Delivery to send an email at 12:01 am the next day. There is no Exchange Server, just Outlook 2007 on the client PC, networked with Windows Server. Instead of sending an email at 12:01 am, an email is sent at approximately 5 am instead. Does anyone have any suggestions on what might be causing the client computer, not to send out the test email at 12:01 am, the next day? The computer is not set to turn off the hard disk etc..