I see from the Kasa app / settings / location and time

that the location can EITHER be set to sync to the "phone's current location" OR entered manually.

This still applies to the whole account, not individual devices.

Unfortunately, therefore, this doesn't completely help the OP @Arubadew - but can perhaps provide clarity after a number of the above replies say it can ONLY be the phone's location.

This does help anyone who has all their Kasa devices in one time-zone yet travels around timezones themselves (with their phone!).

Having bought a Kasa smart device myself based purely on recommendation, I am rather frustrated at it's poor (online/remote) functionality.

@enhompe 

enhompe wrote

In fact I do have Google Homes and Home Minis at each location. Would I then need to have a separate Kasa account for each location?

What I did was to create a Gmail account for my second timezone (California) and a second Kasa account for that time zone. The California Kasa account uses the California Gmail account for the user name. 

This solution works reliably. Night lighting in Thailand is on the correct schedule, etc. 

The only inconvenience is that I occasionally have to log on / log off to switch Kasa aacounts. 

I wish Kasa would introduce a concept of Homes or Locations in the app. But falling short of that, why not implement a pulldown of known Kasa accounts to make it painless to switch back and forth among accounts?

TP-Link folks: take a look at how Google implements this in Gmail. Users tell Google they have other Gmail accounts. For each new one, you login. And subsequently you can simply switch accounts by choosing the account you want from a pulldown.