I am allowed to stay 90 days out of 180.Does the 90 days
start from the first day I arrive in the EU, or is there set dates. So for example my first trip to to Belgium was in July, so does my 180 days expire in January? That would seem the most logical answer, or is there something I need to know?

The 180 days is rolling so you can only ever stay for 90 days in any given 180 day period. Basically if it is more than 180 days since your last visit you are back to 0 days but if you were in Europe within the last 180 days then you need to include that.

Also because its rolling as your current stay goes forward you can discount any day that was within 180 days at the start of your trip but it now over 180 days ago. Im sure once you get your head around it then it is a lot easier in practice to manage than it is to try and explain it

The best way is to download onto your phone the Schengen calculator app then put your holidays in that you have taken the last 180 days and it will continually calculate how many days you have available you can also input the holidays you have booked to go on and it will tell you how many days you have left, its very straightforward happy hols

One of the many benefits of Brexit

From first day - what's the confusion?