Error on OWA and active sync says expired cert, when it's not

Today our remote users when trying to check emails have started to get cert errors stating that the certificate is expired, or not yet valid.  The problem is the certification is valid, and is valid until Aug 17, 2008.  It's issued by THAWTE.  

I've checked the event logs and can't find anything that would be related to this issue.
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Was the clock on the client machine set correctly?
Or maybe the clock on the server?  (Although I would have thought you'd have seen loads of errors if that was the case)

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