The Proxmox VE cluster stack itself relies heavily on the fact that all the nodes have precisely synchronized time. Some other components, like Ceph, also won’t work properly if the local time on all nodes is not in sync.

Time synchronization between nodes can be achieved using the “Network Time Protocol” (`NTP`). As of Proxmox VE 7, `chrony` is used as the default NTP daemon, while Proxmox VE 6 uses `systemd-timesyncd`. Both come preconfigured to use a set of public servers.

| If you upgrade your system to Proxmox VE 7, it is recommended that you manually install either `chrony`, `ntp` or `openntpd`.

## Using Custom NTP Servers

In some cases, it might be desired to use non-default NTP servers. For example, if your Proxmox VE nodes do not have access to the public internet due to restrictive firewall rules, you need to set up local NTP servers and tell the NTP daemon to use them.

### For systems using chrony:

Specify which servers `chrony` should use in `/etc/chrony/chrony.conf`:

    server ntp1.example.com iburst
    server ntp2.example.com iburst
    server ntp3.example.com iburst

Restart `chrony`:

    # systemctl restart chronyd

Check the journal to confirm that the newly configured NTP servers are being used:

    # journalctl --since -1h -u chrony

    ...
    Aug 26 13:00:09 node1 systemd[1]: Started chrony, an NTP client/server.
    Aug 26 13:00:15 node1 chronyd[4873]: Selected source 10.0.0.1 (ntp1.example.com)
    Aug 26 13:00:15 node1 chronyd[4873]: System clock TAI offset set to 37 seconds
    ...

### For systems using systemd-timesyncd:

Specify which servers `systemd-timesyncd` should use in `/etc/systemd/timesyncd.conf`:

    [Time]
    NTP=ntp1.example.com ntp2.example.com ntp3.example.com ntp4.example.com

Then, restart the synchronization service (`systemctl restart systemd-timesyncd`), and verify that your newly configured NTP servers are in use by checking the journal (`journalctl --since -1h -u systemd-timesyncd`):

    ...
    Oct 07 14:58:36 node1 systemd[1]: Stopping Network Time Synchronization...
    Oct 07 14:58:36 node1 systemd[1]: Starting Network Time Synchronization...
    Oct 07 14:58:36 node1 systemd[1]: Started Network Time Synchronization.
    Oct 07 14:58:36 node1 systemd-timesyncd[13514]: Using NTP server 10.0.0.1:123 (ntp1.example.com).
    Oct 07 14:58:36 node1 systemd-timesyncd[13514]: interval/delta/delay/jitter/drift 64s/-0.002s/0.020s/0.000s/-31ppm
    ...