Of course we all know about the Schengen rules, and I always thought I understood them, but I never really had to pay close attention, as our visits never exceeded three weeks every year or two. Now that we're retired, though, we're hoping to extend our stay. We've read the rules on the website, but we are just not clear how to interpret the rule as applied to our situation.
We visited Portugal for 31 days, April 25 to May 25. It is our understanding that our "180 days" began on April 25th. Now, we are returning, this time to Spain, arriving October 8. Would we be correct in our understanding that the 180 days period would end on about October 25?
Here's our first question: Must we exit Schengen area on October 25, or does a "new" 180 days begin after that? If we exit, how long must we be outside of Schengen before we can return and begin a new 180 days?
An alternate interpretation might be that 180 days begins October 8 upon our arrival in Spain?
We wish to adhere to the rules, but we get more confused each time we consider them! Does anyone have a clear understanding to help us clarify? Thank you so much!

You have 90 days in any 180 days. So your clock started on April 25th and counts until 180 days later. During that time you have 90 days available to you. You must then leave Schengen. It does not start the clock again until you have been out of Schengen for 90 days, at which point you have another 90 days in 180 in Schengen.
There are calculators online to help you work it out.
You must also have enough time on your passport on the date you leave.

The way I read your post you will NOT have exceeded 90d in the 180d period from 4/25 to 10/25 so there is nothing forcing you to leave the Schengen zone. It sounds like a new 180d clock will start on 10/8 for you. - but i agree with h - use one of the online calculators to run the #'s.

The 180-day clock is a rolling period, an ever-changing window that allows you to stay for up to 90 days in any 180, consecutive day period. Every day is a new calculation which is why an online calculator is the best way to compute eligibility.

Definitely check out the online calendars. But what I do is, count backwards. The day you are leaving Schengen, count back 180 days and make sure there are not 90 days in Schengen in those 180 days. As Sarasto says, it is a rolling calendar now and the calculators are really helpful in figuring everything out. (I always do my counting backwards, just to be in the safe side, but unless you are really filling up your Schengen days you probably don't need to do that.)

Here is a good calculator - https://schengen-calculator.com/

Thank you for reply - definitely helps. We went to the online calculators (one worked better than the other for us) and plugging in a couple different scenarios clarified this for us. Thank you ;=)

Thanks for the help!

Thank you!