It should be implemented in people's brains.
How this goes, usually, is:
Them:...before 6PM.
Me: 6PM... Ours? The server's? The user's?
Them: GMT, of course.
Me: So that's 5PM London right now, and changes to 6PM in November?
Them: What no are you stupid. Always 6PM GMT.
Me:* jumps off a cliff*