I would stick with just 24 hour clock, its not hard for someone to add 12 to the hours if its pm, if it is then there scripts likely to fail every where else as well LOL
/timer -someSwitch 12:30 0 somealias
3 ways i can see this.
(1) /timerXX 12:30 0 86400 somealias | /timerXX.oneoff 12:30 1 0 somealias
or
(2) /timerXX 12:30 1 0 somealias $(|,) timerXX 0 86400 somealias
or
(3) /timerXX 12:30 1 0 somealias
somealias has this /timer command in it on the first line so sets it up again* 3 is the most exact accurate but takes work on the alias to setup (or a wrapper alias which calls the real somealias thats needed and set the timer)
* 1 is what I use mostly
* if you watch the timers list when you have a
/timerXX 12:30 1 0 echo its 12:30! wehn 12:30 hits it actually makes another timer
/timerXX 1 0 echo its 12:30!