The rule is the "day" before, not 24 hours.

CO starts running their EUA at midnight IAH time. It usually takes 2-3 hours for it to complete. From what I've seen, it appears flights are hit in order of absolute time of departure (e.g., 7:00 a.m. out of DEN before 6:01 out of SFO), starting with the 12:xx a.m. redeyes, and ending with the 11:xx p.m. ones.

NW starts their EUAs a few hours earlier than CO. It's not uncommon to get a NW EUA at 6pm-ish Pacific Time, two days prior to departure for a Silver.