I was not able to find an API that fully disables time sync. This blog does have some ways to do it that could work: Completely Disable Time Synchronization for your VM - Virtualize Business Critical Applications - VM...
I had a look into API calls and I did not found any section which tells about disabling/enabling/changing time settings.
I made API call against top of the VM and was hoping to get something for time settings, but did not found any.
curl -sik -H "Accept:application/*+xml;version=5.6" -H "x-vcloud-authorization:Auth" -X GET https://vCD-FQDN/api/compute/api/vApp/vapp-87ec9f6d-75e1-4556-b5e9-3a2ac10a7228
Also I checked API Admin and User guide and did not find any mention of this feature.