Originally Posted by
corporate-wage-slave
5) Yes, midnight for the departure airport.
A little extra data in case it helps anyone...
I can't generally change my corporate TA issued tickets online, and anyway had already checked in for my 07.30 trip. Trying to do SDC when the UK office opened at 6am seemed a little risky, but a call to BA in New York after midnight UK (but before US close at 1am UK) had me offloaded and moved to a later flight with no fuss or phaff ^. The agent was aware of the same day change rule or saw it in the booking rules, although they presumably don't get a lot of call to use it in US.