My cisco (router or switch) send the current date via NTP multicast message, I use the following configuration :

!
ntp peer 192.168.34.3
ntp source Vlan34
ntp authentication-key 1 mdr MY-SECRET-KEY
!
interface vlan34
  ntp multicast key 1
!

/etc/ntp.conf

restrict default noquery notrust nomodify
restrict 127.0.0.1

keys /etc/ntp.keys
trustedkey 1
multicastclient 224.0.1.1


driftfile /var/lib/ntp/ntp.drift

/etc/ntp.keys

1 M MY-SECRET-KEYS

Check if the ntpd accept the multicast messages :

# sudo ntpq -n
ntpq> pe
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.1.252   192.168.34.3     3 -    1   64  377    1.143   -4.698   0.173