First, make sure your system has the right time itself.

You should therefore have the ntp or ntpd package installed. Now simply add a restrict line without nopeer nor noquery against the subnetwork you wanna allow e.g.,

vi /etc/ntp.conf
restrict mask nomodify notrap
enable mode7

Don't forget to setup the local clock as a fail-over, if internet connection goes down, so you still provide the service,

server # local clock
fudge stratum 10

#server     # LCL, local clock
#fudge stratum 12  # increase stratum

restart ntpd and call the local daemon (mode7),

ntpdc -l
ntpdc -c sysinfo