How to trigger NTP client to sync time?

I have an internal NTP server in the LAN (so there's no need to configure any firewall for this matter) but my ESXi 4.1U1 servers are totally out of time (3 to 5 minutes).

I've tried to restart the NTP client n times but the time is still not synch'ed.

Of course, I could manually change the time but what's the point to have the NTP client if we have to do it manually?

Is there any command (through SSH) to trigger the time-sync?

TIA