I think you need to look at the offset, rather than the timezone name. For example, if I fly to somewhere in e.g. the Central Europe timezone I don't always pic

I think you need to look at the offset, rather than the timezone name. For example, if I fly to somewhere in e.g. the Central Europe timezone I don't always pick a city in the same country when setting the timezone; I just pick Paris or Berlin or whatever comes to mind first. But then if you only look at the offset, you'll have even worse VPN detection since any VPN exit in the same timezone offset won't be detected. This is not really a good way to detect VPN.