Is is possible to check in for tomorrow's flight before today's flight leaves?

My wife asked me to check her in for her return flight tomorrow at 3:30p. Her flight today leaves at 4:25p.

The system kept telling me the flight was leaving in less than an hour (for today's flight), so checkin was not possible for tomorrow.

As soon as today's flight departed, it let me checkin for tomorrow's but not a minute sooner.

Is this the way it is supposed to work? What happens if you have multiple flights in one day (not a connection, a new flight?).

Does anyone know?