Which version of znc do you use? Maybe your znc conf ist broken. Look at the version.txt of mirc 7.34:

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.


cu

TC / Mario