ntp won't work on read-only Raspberry Pi

I found this question while facing a similar issue.

The issue turned out to be that systemd's PrivateTmp feature does not work in a read-only configuration.

1. Be sure to install ntp and ntpdate

   sudo apt install -y ntp ntpdate
   

2. Copy /lib/systemd/system/ntp.service to /etc/systemd/system/ntp.service

   cp /lib/systemd/system/ntp.service /etc/systemd/system/ntp.service
   

3. Open /etc/systemd/system/ntp.service and comment out PrivateTmp=true.

   sudo nano /etc/systemd/system/ntp.service
   

Now, it should work correctly!

As an additional step I have also now mounted /var/lib/ntp as tmpfs as recommended here

4. Open /etc/fstab and add tmpfs /var/lib/ntp tmpfs nosuid,nodev 0 0 at the end of file.

   sudo nano /etc/fstab
   

I didn't find this necessary in my case but there are additional insights into running on a read-only filesystem there.