This would be interesting to add. I don't use any JavaScript, so I would have to get the user to set their time zone and the time after which they want to switch.
The prefers-color-scheme CSS media feature could be a way of doing this without requiring any additional action from the user. The spec is still a draft but it has decent browser support.
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...