what's the measure for time here? ms, s, ticks?

Alarms use ingame time of day. Ingame time of day is a number in the range [0.0, 24.0) that does not have a unit.

EDIT: Tho if one really wanted they could consider it ingame hour?

As @Wojbie mentions, this uses in-game time, which is effectively hours. I've added a link to os.time in e188f1d, which explains this a little better.