You look to the time, because something irregular seems to be happening outside the normal schedule.

臨時 highlights something out of the ordinary, like 臨時の仕事 (temporary job) or 臨時休業 (unscheduled closure). It can also sometimes mean "extra," but only in a special, irregular sense, like 臨時ボーナス (extra bonus).

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.