From http://www.total-knowledge.com/~ilya/mips/ugt.html

UGT […] states that it is always morning when person comes into a channel, and it is always late night when person leaves. Local time of any member of channel is irrelevant.

Great convention for groups of people who spend time travelling across timezones.

Unfortunately the habit sticks, and now I find myself having to explain an oddly-timed “good morning”!