Nope, the "standard" format is in seconds. And the first "1" is the number of iterations/repetitions of the command.
But you can set the "delay" in milliseconds by using the -m switch. Half a second would be 500ms, therefore change the line:
.timer 1 1 describe $chan don't listen to $nick
to:
.timer -m 1 500 describe $chan don't listen to $nick
EDIT: RusselB beat me to it. 
