Outlook Meeting Planner TimeSync Problem

Have a strange problem on Outlook (2003): with one of our suppliers, whenever we get a meeting request, the meeting time is 1 hour out on our systems, although correct on theirs. This only happens with this supplier. On their side, they have one other customer with whom this happens, other meeting requests go through with the right time. The systems in question are all set to the same time zone in Microsoft. Has anyone else encountered this issue?