Irrespective of the “Synchronize clock with ESXi host” option kept disabled or not on the VM which is the default setting, there is an extra property of VMtools that could trigger a time drift to match the clock on the host on which it resides.
You would have one of these scenarios on the VM which would have caused that time drift.
1. Resume a virtual machine from a suspended state.
2. Take or restore a snapshot.
3. vMotion a virtual machine
4. Shrink a virtual machine’s disk.
5. Reboot a virtual machine.
6. Restart the VMware Tools service on a virtual machine.
7. Applications syncing to an alternate NTP Server in another time zone BST, GMT etc, location.
During operations listed above, VMware Tools will adjust a VM’s clock to match that of the ESXi on which it resides so the likely chance here is the esx host will be syncing to a different time server & your vDC will be syncing to some alternate ntp provider.
There are tweaks you could do prevent this sync in active directory vDC VM.
Read pg-21 of this document.
https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/Timekeeping-In-VirtualMachines.pdf