I think you may be taking the song a bit too literal (especially since you quoted the "somewhere" in your second paragraph).
It is a song about drinking any time of the day...
Maybe you've been living out the songs message and aren't thinking entirely straight.
Regardless, here you go: the 5o'clock refers to a method of keeping score. A day, hour, minute, etc is of arbitrary length, only having the value we agree to assign to it.
Because time zones (used to keep the sun generally rising and setting around the same time across the globe except in the extreme north and south) are differentiated in blocks larger than 1 minute (typically 30 or 60 minutes) it is not 5o'clock somewhere when it is 5:42 somewhere else.
However, the sun's location across a time zone will be higher in the sky as you move further west. So for example, in eastern Arizona (when it is on Pacific time) the sun will set earlier than it does in California. 5o'clock in Arizona may look more like 5:42 in California.
Enjoy my much more lengthy than necessary answer. I am sitting at work with nothing to do. So you get to read my ramblings.