Poseidon hadn't much liked being interrupted with his day to go see Zeus. It seemed awfully rude of his brother to demand his time when he had other things to deal with this Monday. He kept Theseus in charge for the remainder of the day (whether he wanted it or not). It would make the boy earn his keep, learn the ropes of his old man's business. He had a chance to prove he was more than just a washed up hero.
He showed up in his own time, he didn't rush over immediately. If Zeus wanted to infringe on his time, he wasn't going to hurry. He pushed the doorbell. This would go over well.