The protocol contains times and dates in various places.
For the -D option to the annotate, co, diff, export, history, rannotate, rdiff, rtag, tag, and update requests, the server should support two formats:
26 May 1997 13:01:40 -0000 ; RFC 822 as modified by RFC 1123 5/26/1997 13:01:40 GMT ; traditional
The former format is preferred; the latter however is sent by the CVS command line client (versions 1.5 through at least 1.9).
For the -d option to the log and rlog requests, servers should at least support RFC 822/1123 format. Clients are encouraged to use this format too (the command line CVS client, version 1.10 and older, just passed along the date format specified by the user, however).
The Mod-time response and Checkin-time request use RFC 822/1123 format (see the descriptions of that response and request for details).
For Notify, see the description of that request.