It's been wonky for me too. I try to schedule releases in advance, but more than two days out and the date changes, not just the time. I've given up trying to schedule too far in advance. I did figure out the GMT time thing, but it still gets way off when you try to schedule too far in advance. For instance, if scheduling for tomorrow, I can tell it to post at 1300, which is 7 am Central. No problem, I can tell it to post the next few days at 1300 and the chapters should all post at 7 am Central / 8 am Eastern and so forth. But, get more than two days out and the date will change at random. It is therefore impossible to schedule chapters very far out. Editing the draft and changing the date by hand makes no difference.
No big deal, but it would be nice to post farther in advance than a couple days out.
Edit - I see now after playing with it more, the date jump occurs on account of the time difference. It will go into the following day if you schedule too late in the evening. That threw me for a loop.