Time synchronization within the domain is essential.
How can I keep my time synchronized across the domain when some or all Univention UCS hosts are running as virtual machines (Proxmox, vmware, Qemu, any)?
By default the hypervisor syncs time to its virtual machines only at specified occasions (see here for vmware)
UCS installs the NetworkTimeProtocolDaemon (ntpd) which offers the local host time to the domain. The ucr variables timeserver, timeserver2 and timeserver3 can be used to add external timeservers as references.
Domain clients like Windows sync their time to the domain controllers (Win AD or UCS).
Use only a single time synchronization option, do not use multiple sync sources as this might confuse the internal ntpd of UCS.
Option 1: Enable time synchronization from the hypervisor (ESXi, …) to be synced regularly to the virtual machine. In this case do NOT set additional external timeservers through ucr variables!
Option2: Set timeserver variables in UCS to sync to an external ntp-server (local or internet, ideally use multiple ones). Disable sync through hypervisor then (will be synced on power on, though).
UCS systems by default install ntpd and synchronize to the UCS master. The mentioned ucr variables define additional servers from where they will sync time.
For domains I would suggest to disable hypervisor sync and enable external time sources at least on the master. Sync the backup, slave or member servers to external sources is not necessary, but possible.