From communicating with enough people like that, I've learned to be horribly specific, to the point of appearing anal, just because I know that people can and do misinterpret these kinds of things. I repeat things back to people all the time, even though I fear it comes off as obnoxious.

So, while I don't think there's anything wrong with the way you communicated, I would have gone the extra mile and said something like, "Yes, Sunday morning is fine. Please have it in my inbox by 8 a.m. Eastern time" (or something like that). Just because I've had too many experiences with people who will glom onto any ambiguity and exploit it (consciously or not). Ugh!