$msgstamp will return the time, you could compare it to $ctime but its presence at all should be enough.

http://www.mirc.com/versions.txt

Originally Posted By: versions.txt
53.Added CAP server-time and znc.in/server-time[-iso] support. Enabled
automatically with a CAP request if server lists it in CAP LS. If a
server message has a @time prefix with a UTC timestamp, $msgstamp
will return this value.