I can't figure out a way to have mIRC do this. when you specify a time, the only thing I know of to try is to set it to trigger at say midnight, and delay for 86,400 seconds so it happens every day at midnight, but it's off by a few seconds daily. By about three weeks later it was over a minute off (mIRC and my computer are on 24/7).
I think this needs to be a new feature to the /timer in mIRC 6.04