Welcome to iPhone Forums!
Go to the Settings app, select General - Date & Time, and disable „Set Automatically“.
Wait for it... Only answer I have. Mine did that too for awhile. It kept switching to some time zone I never even heard of. It sorted itself out after some time though. I was never able to force it back and keep it back. Kind of did it on it's own. I heard it had something to do with the cell tower near my job sending wrong signal, but no idea if that's accurate or not. All I know is it fixed itself.
EDIT: I just noticed this is an old post. My mistake haha.