We have many servers, all of which sync their clocks with one "primary" server (SV1).
The clock on our Exchange server (SV2) occasionally gets out of sync, typically by 60 seconds exactly. SV2 is now set to sync every 15 minutes, but this does not seem to help.
SV1 is Window Server 2008 R2
SV2 is Windows Server 2003
This network is not on the Internet, so there is no syncing to public time servers.