Best practices for NTP updating in VirtualBox virtual machines without guest additions?

I have a couple of VMs that cannot have guest additions installed, and I pause and resume them with days between. I normally forget to nudge NTP when I start up the VM. Is there a good way to tell ntp to be very aggressive in how often it should check for changes?

I know you said you couldn't install them, but it's worth saying that the machine additions were created (at least partially) to solve this problem; and should be the solution.

ntpd can only be set to ignore wild offsets at startup; so you'll have to configure the machines to run ntpdate via cron or similar.

The Vmware suggested NTP configuration for a similar issue will almost certainly apply here.

You need to set the tinker panic 0. Make sure your system is not using the local clock as a high stratum server.