Its a general saying that is used to greet/say goodbye to people that live in a different time zone than you.
Its said all together to reach everyone and not just one person
Typically used in online conversations&mass text
Jane Seymour:i wanna tweet/text everyone goodmorning but i know were all in different time zones....hmm.."
Helen Mirren:"Try gd'mornafternoonight"
Jane Seymour:Genious! That way its a greeting to everyone besides hi or bye....its both!"