I flatter myself to think that I've got the hang of the eight-hour time difference between here and the UK - not that that stops me beginning a family Skype conversation with the words "What's the time over there, then?". That said, cringeworthy mistakes do still happen, and not even their initial concern that you've been run over will protect from the wrath incurred by a 3am phone call.
To this end, I heartily recommend that any Gmail user in exile from the homeland enables the Sender Time Zone feature in Gmail Labs (ie. the thrilling antechamber where you can test-drive probably-great-maybe-rubbish beta propositions on your Gmail account). Not only does it show the local time an email was sent, it also gives the current time over there. Probably the most useful element is the little phone icon that appears next to a sender's name, which turns green during local 'business hours' (9am - 6pm) and red-for-danger outside of those. Whilst I'd argue they could afford to extend that to 9pm (which, strangely enough, this otherwise-useful Lifehacker article seems to believe they already have done), it's preposterously useful as an at-a-glance guide. Just make sure you've got your correct time zone set in your own Google account settings, obviously.
Sadly, the biggest issue still remains. Tell me - when IS the 'cut-off'?