On Windows 2003 it is possible via the registry to set a custom interval for updates.
This is done via the following registry key:
HKLM\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval
After adjusting this key and then restarting the Windows Time service, when I update the time changes to (for my setting) one day from the time I clicked the update button.
What I don't know how to do is set the exact time at which the update occurs?