Returns the number of seconds since the unit was last changed state or value, the last time a global script was executed and as of build 834 the seconds since an interface has last received a packet.

time delta of (name of Unit, global script or interface)

write log "it has been " & (time delta of "Kitchen Motion") & " seconds since motion in the kitchen"

Note that this will also work for global scripts to tell you the last time the script was ran, or with interface names to tell you the last time an interface received or sent a command.