Many options take a date argument. CVS accepts a wide variety of date formats – too many to list here. When in doubt, stick with the standard ISO 8601 format:
This means "23 August 1999" (in fact, "23 August 1999" is a perfectly valid date specifier too, as long as you remember to enclose it in double quotes). If you need a time of day as well, you can do this:
"1999-08-23 21:20:30 CDT"
You can even use certain common English constructs, such as "now", "yesterday", and "12 days ago". In general, you can safely experiment with date formats; if CVS understands your format at all, it most likely will understand it in the way you intended. If it doesn't understand, it will exit with an error immediately.