Thanks everyone for the help!? I agree that Zoho should be a bit more lenient on time being off by less than a minute.? I have engaged them again on this and await their response.
?Mark reports that it should be fixed now.
He's a bit mystified ?as to why ntpd wasn't running on a couple of machines, even though they were set up exactly the same as the others (where it was). He's updated the setup scripts to (hopefully) prevent this from happening again.