Acme packet SBC is synchronized with NTP server but its clock doesn't sync with NTP server

Hella Everybody,

I want to sync SBC time with NTP server so I did ntp-syn configuration on my SBC like below:

#conf t

#ntp-sync

#select

#add-server x.x.x.x ( my NTP server address )

when I type show ntp status and show ntp server, I can see SBC synchronized with NTP server.

But show clock shows different date with my NTP server approxymately 2 minutes.

So I want to if there is any other command that I need too enter on SBC to force this synchronisation.

Thanks in advance.