It would be easier if you had included a copy of your formula, but my guess is your using 

Hi @BillH ,

Thanks for your help. You are right, I should have shared my formula.

Is exactly what you suggest (DATEADD({Fecha},1,'year')) and gives 23/12/2024 as a result.

If I test DATEADD({Fecha},365,'days') the result is 22/12/2024 😂

Any advice?

Thanks again

Since it works fine on my base I did some more testing.  My formula field was formatted to display using gmt time, if I change it to local it displays the incorrect date as in your example.

So, as a quick fix you might try looking at the formatting and switching to gmt.  I know this is not a real solution, but it might work for the immediate concern.  There have been multiple discussions on this forum discussing how airtable handles dates and providing workarounds, and I'm sure someone has a better long term solution.  Hopefully they will respond.

If I have more time later in the day I will get back to this.

I knew Kuovonne had an explanation better than mine...

https://community.airtable.com/t5/formulas/dateadd-formula-isn-t-working-correctly/td-p/38577

Yes! That's it!

Thanks a LOT @BillH