If you have Mitel Mial or NuPoint voice mail system, the time settings are not linked to your PBX.
You have to log into your system via either a web browers or Telnet (200 ICP) or via a Laptop/Terminal into the front maintenance port.
Login to maintenance then hit the "system" button and then "set" button. ( I'm doing this from memory but I think the buttons are close enough to get you there.)
Ralph