staying in Europe over 90 days

You'll be able to make this work, but be very, very careful. Both arrival day and departure day count, and you're going in and out of the Schengen Zone twice. The day you leave for England and the day you return from England both count, as do September 3 and December 12.

If my count is correct, there are 101 days between September 3 and December 12 inclusive. That means you need to be totally outside the Schengen Zone for 11 days. Counting the arrival and departure days for England, you need to plan at least a 13-day trip. I would never cut it that close. What will you do if one of you gets ill or there's a catastrophic problem affecting transportation on your planned travel day? The same thing could happen on December 12, of course.

I don't blame you for wanting to avoid the visa hassle, but do be conservative in your scheduling. My last Schengen stop in 2015 was Slovenia. I went to Ljubljana on about Day 82, planning to stay there for a few days then head to Piran before crossing the border into Croatian Istria. I became ill and barely got out of Slovenia on Day 89 or 90. That was a close call.

This summer I traveled by ferry from France (on Day 89) to the Channel Islands, planning to visit Jersey for a few days then continue on to England by ferry. The ferry out of France was fine (thank heavens), but the ferry out of the Channel Islands was canceled for three days in a row; fortunately, I was already outside the Schengen Zone by then. Close Call #2.

It has been reported that overstays can lead to penalties in excess of 1000 euros and being banned from the Schengen Zone for multiple years.

Thanks, this is good to know. We haven't purchased anything yet so can flex dates to make this work. I think Hungry, Poland, Chech Republic and Croatia are all Schengen Countries. So I will count my days!

With the exception of Croatia. Croatia won't be in Schengen before 2019 (at least).

Yes, you can exclude Croatia for now.

Thanks. What about Slovakia? Can I expect countries not in Schengen to always check my passport? Just wondering how I'll prove exit and reentry?

Slovakia is Schengen.

Someone suggested recently that you take some Post-it notes with you and use them to mark the passport pages with entry and exit stamps. I think that's a very good idea. You don't want to miss a flight while the immigration officer fumbles through your passport.

https://europa.eu/european-union/about-eu/countries/member-countries_en

Helpful info there

Slovakia is in Schengen and on Euro.

... I'll prove exit and reentry?...... By making ABSOLUTELY CERTAIN you have clear entry and exit stamps. I am the one who made the suggestion to mark with sticky notes or a paper clip the page with the entry stamp. I had a very nervous five minutes or so when a French immigration office could not find an entry stamp for my last entry. He had found a nine month old entry stamp and he was keeping his thumb on that page.

90 non consecutive days in Europe of the 180 day period You have the right idea but that technically is not completely correct. It is 90 days out of any prior 180 days. Everyday you spend in the Schengen you start another 180 day period. It is a look back. There is never a reset.

Do you already have your plane tickets? Is your time in London and(?) England tied to something specific there in October?

If nothing is paid for yet and you are flexible, it might be easier, more efficient and more organized to do a multi-city flight itinerary and put England at the beginning or end. Then sort out all your other countries, find a beginning or ending place and fly to or from there as needed.

Planning that way might help avoid a costly overstay mistake and perhaps allow for more quality time in the Schengen countries because there'd be fewer trips out of and back into them using up Schengen days.

Many ports of entry to the Schengen don’t stamp any longer. Last several trips to Croatia, for example, we were not stamped. Everything is computerized now, so less stamping and better reason to keep your own records.

Thanks for posting that, Tom. As I read it, you can enter Denmark from any other country, but time spent in the other Nordic countries does count against the extra 90 days you can spend in Denmark.

I didn't notice the exit-only-through-Copenhagen-airport restriction, though. Where did you see that?

I see what you mean. The Danes wouldn't care (presumably), but the next country you hit by car/bus/rail/ferry definitely would.