DHCP servers can broadcast the NTP server for clients to use. Here's how.

1. echo 'dhcp-option=42,172.16.0.9' | sudo tee /etc/dnsmasq.d/42-pihole-dhcp-ntp.conf
2. sudo service pihole-FTL reload
3. That's it!

To test:

sudo nmap -sU -p67 –script dhcp-discover 172.16.0.2

PORT   STATE SERVICE
67/udp open  dhcps
| dhcp-discover:
|   DHCP Message Type: DHCPACK
|   Server Identifier: 172.16.0.2
|   IP Address Lease Time: 5h58m52s
|   Subnet Mask: 255.255.252.0
|   Broadcast Address: 172.16.3.255
|   Domain Name Server: 172.16.0.2
|   Domain Name: syninf.local
|   NTP Servers: 172.16.0.9
|_  Router: 172.16.0.1