Yes, thanks for the correction. I clearly shouldn’t do time math in public.
To further confuse matters, I took the option in Gmail’s triple-dot “more” composition menu to “Set up a time to meet” hoping it would include a useful calendar attachment or at least a link to include the event in a recipient’s Google Calendar. As received by another account of mine, it was neither, and the text had the wrong time (13:00 UTC) despite having the correct time in my Google Calendar, which is set to show both UTC and Eastern time. Local time can be tricky, and I’m glad NTP generally shrugs and leaves that problem to others.