Set a variable %oldctime when the event occurs to $ctime, and check before this, that the variable exists, to announce the time since the last run, let's suppose the event is a simple on text:
on *:text:!newevent:#:{
if (%oldctime != $null) {
var %diff = $ctime - %oldctime
msg $chan Last event was on $asctime(%oldctime) -- time elapsed since: %diff seconds, or $duration(%diff)
}
msg $chan New event!!
set %oldctime $ctime
}