timesyncd

As a general rulle, keeping all your hosts time in sync is a good thing.
Linux variants running systemd will typically use timesyncd for this.

Check the status of timesyncd

# timedatectl timesync-status
       Server: 91.189.89.198 (ntp.ubuntu.com)   
Poll interval: 34min 8s (min: 32s; max 34min 8s)
         Leap: normal                           
      Version: 4                                
      Stratum: 2                                
    Reference: 11FD6C7D                         
    Precision: 1us (-23)                        
Root distance: 32.569ms (max: 5s)               
       Offset: +68us                            
        Delay: 14.613ms                         
       Jitter: 41.638ms                         
 Packet count: 256                              
    Frequency: -21.021ppm                       

Note that the NTP server being used is the default for an Ubuntu install, ntp.ubuntu.com
This particular server is running a lot of LXC containers, one of which runs CCTV software.
As it turns out, RTSP streams for IP cameras are time sensitive. If the sending and receiving host times are not in sync then there will be problems with the RTSP feed, e.g. Invalid data found when processing input.
Containers are prevented from running timesyncd by the service configuration, ConditionVirtualization=!container so they get their time from the host.
While this particular host was running timesynd and it appeared to be working the time was still drifting enough to affect RTSP streams from the container.
Updated the timesyncd configuration and changed the NTP servers which appears to have fixed the problem.
Before

# cat /etc/systemd/timesyncd.conf

[Time]
#NTP=
...

After

[Time]
NTP=0.pool.ntp.org 1.pool.ntp.org 2.pool.ntp.org 3.pool.ntp.org

Restart timesyncd and check the status

# systemctl restart systemd-timesyncd.service

# systemctl status systemd-timesyncd.service
● systemd-timesyncd.service - Network Time Synchronization
     Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled)
     Active: active (running) since Sat 2020-11-14 09:43:16 GMT; 6s ago
       Docs: man:systemd-timesyncd.service(8)
   Main PID: 3666655 (systemd-timesyn)
     Status: "Initial synchronization to time server 217.114.59.3:123 (0.pool.ntp.org)."
      Tasks: 2 (limit: 4620)
     Memory: 1.6M
     CGroup: /system.slice/systemd-timesyncd.service
             └─3666655 /lib/systemd/systemd-timesyncd

Nov 14 09:43:16 vh1 systemd[1]: Starting Network Time Synchronization...
Nov 14 09:43:16 vh1 systemd[1]: Started Network Time Synchronization.
Nov 14 09:43:16 vh1 systemd-timesyncd[3666655]: Initial synchronization to time server 217.114.59.3:123 (0.pool.ntp.org).

# timedatectl timesync-status
       Server: 217.114.59.3 (0.pool.ntp.org)   
Poll interval: 1min 4s (min: 32s; max 34min 8s)
         Leap: normal                          
      Version: 4                               
      Stratum: 2                               
    Reference: 9D2CB004                        
    Precision: 1us (-20)                       
Root distance: 23.894ms (max: 5s)              
       Offset: +844us                          
        Delay: 21.315ms                        
       Jitter: 0                               
 Packet count: 1                               
    Frequency: -14.420ppm