I've a cluster VSphere 4.1 with many host with HA and DRS enabled. I find that every ESX host clock differs in time configuration by each other ... the time differences in some case is also same days ... fortunately all virtual machines aren't syncronized with the host ...
Now if i'm starting to modify clock of each ESX, like setting ntp, stop service, ntp restart, or configuring it manually. Can I have some trouble? Or may attention in something?
If you have running NTP servers and setup your hosts for it, dont forget to enable time synch with host in the VMs VMware Tools. That will help you a lot later
You should be aware that even if you aren't synching time with the guest, certain operations will always cause a time sync. The two that come to mind are taking/deleting a snapshot and vMotion operations. Even if time sync is disabled in the VMware Tools, the guest will still sync time with the host after those operations.
So yes, you should make sure that your ESX are using the same NTP time source so that they all stay in sync.
Here's a good article on timekeeping best practices for virtual machines: