Good job realizing that you were heading down the path of an argument. Sometimes it is just not worth the headache. If you have not done so already, maybe post a calendar in a location where he can see it so that you don't have to constantly remind him (I know that my H hate nagging). If he continues to be unreliable and lie, then maybe you need to just make plans as if he is not going to help. If he asks, just say that you have no problem including him in conversations regarding logistics if he is going to be let you know what he can and can not cover and will actually follow through on his commitments. Otherwise, you need to figure out a plan to make sure everything is covered.
My H used to be the exact same way. I started getting people to cover to make sure that someone would be there to watch the kids. I stopped asking him because he was so unreliable. Eventually he wanted the time with the kids. I said no problem so long as you don't back out. He now puts everything on his calendar, shows up when he needs to be there. He has stepped up and I no longer have to nag/argue.
