UK with US Passport

If US citizen you can do that. Not sure what you are asking about the clock in UK -- it is six months. Just like the Schengen zone. It is a look back so it is six months out of the prior 12.

Starting January 8, 2025 you will need to get an ETA (Electronic Travel Authorization) to visit the UK. Cost is 10 GBP and you can apply starting Nov 27.

https://www.gov.uk/guidance/check-when-you-can-get-an-electronic-travel-authorisation-

For Schengen, it is called ESTA and that will come into effect mid-2025.

The link above has it's URL truncated- this is the correct link- https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

For Schengen, it is called ESTA and that will come into effect mid-2025.

That should read, is currently scheduled to come into effect- it has been postponed many times before, and may well be again,

ESTA is for travelers to USA.

ETIAS is for travelers to EU. Currently rumored to start sometime in 2025. Go only to the official “.eu” website to apply when it’s finally up and running.

Happy travels!

It's a very strange rule in the UK. When you enter, you get up to six months.

However, there is no rule as to how long you have to wait to come back.

Since the beginning of June, I've been in and out of the UK twice. Each for two weeks. When I entered both times, I was able to use the egates with no problems.

With Schengen, you get 90 days in a 180 day period. So it's not 90 days in each country. It's 90 days total in the Schengen area.

Not sure how things will change with the ETA.

"Not sure what you are asking about the clock in UK -- it is six months. Just like the Schengen zone. It is a look back so it is six months out of the prior 12."

This is incorrect - as a standard you receive six months on each entry to the UK; there is no 'clock' the way there is for the Schengen Zone.

https://www.gov.uk/standard-visitor

From the above link-

you’ll not live in the UK for extended periods through frequent or successive visits, or make the UK your main home

So there is no hard and fast rule. But if you stayed in the UK for a very extended period (but under 6 months), left for a short time and then came back again then Border Force may at their discretion take a dim view.

Six months in twelve was an old guideline (not a hard rule) to assess whether people were attempting to live more or less permanently on visitor admission. It no longer appears in official immigration documentation but could live on in some border officers' approaches if involved.