computing the averages for every quarter from monthly data for further calculations

Show us what you want. Mock up a few rows of sample results data as you want to see it (e.g. for the first five years). Forget the PT for now.

OK - using the pivot table in your workbook, next to it (to the right) please manually mock up what you mean by this:

From this value I want to substract the maximum value which was in the interval from 2nd quarter 2002 until know (sic).

I am struggling to visualise what you want. Highlight the cells you'd want to include in the calculation.

Define "the last data"? Which data is this in the workbook?

I understand the calculation you want to make, I'm just not quite getting which data to include.

It's a shame that you cannot calculate ONE example manually to illustrate your requirement, as this would obviate doubt.

I'm sorry, I don't understand.

I am sorry that you feel that my post is in any way impolite or inappropriate: please tell me why you think this is the case. Maybe you have misunderstood a word or phrase I used, but there is nothing impolite or inappropriate about it.

I am trying to help you but I need to establish exactly what you are looking for, and I have not yet fully understood this.

I also see that you have awarded negative rep. This should only be used if someone has insulted you, and that isn't the case here. I am flabbergasted by your response to an offer of help from a forum Moderator!

Maybe this is because I'm not an English native speaker.

Clearly that was it. I was a teacher before I retired (of French and German, as it happens), but I was never a mean one. I am not that sort of person. I am quite upset that you have jumped to conclusions without even establishing the meaning of what I said. Don't do that again, please - it is very unfair and really quite unkind. And never forget that we offer help because we want to help you.

It would be nice if you would undo the unjustified negative rep you gave me by giving me some positive rep.

I have to go out now, so hopefully someone else will be able to assist you.

Good to know the expression "shame on you" is not rude for English native speakers But anyway, thank you for your willingness to help. I appreciate it. Unfortunately, I cannot undo this. I have a notice: you must spread some reputation. Once I am able to do it, ofc I will undo this

That would indeed be very rude for English native speakers, but I did not say "shame on you"! Read the post again. You had said that you couldn't calculate an example to demonstrate what you wanted, so I said:

It's a shame that you cannot ...

This is roughly the equivalent of "Schade, dass ...", NOT "Schäm dich!" It merely expresses regret, nothing more.

Flipping heck! (Look that up if you don't understand it).

I am back home, but tied up at the moment. If I have time, I'll have another look at your issue later on.

OK - so the data in your workbook goes from 01.01.2001 to 01.07.2013.

and I have values for these monthly data, I need for every quarter for every year the averages.

I understand this.

Then when I have the averages I want to take the last value. It would be the 3rd quarter of 2006.

Why would this be the last value? How would I know this when the data runs on into 2013?

And I want to take averages from the first quarter of 2002 until the 3rd quarter of 2006,

That's fine, but where in the workbook are these parameters set? How have you decided on the range from Q1 2002 to Q3 2006? This is where I am confused.

and from these values choose maximum value.

Understood.

Once I have the last value from the 3rd quarter of 2006 and the maximum value from this interval I want to subtract them.

Also understood.

Yes, but example data has to reflect reality.

So, in the sample data, the last value is in fact 01.07.2013 - yes?

I still don't understand why we are starting with 2006. Where has this come from?

Some sample output would help to clarify exactly what you want - thanks.

I hope you have read post #12. Because you need to.

Emilia - please don't be impatient! I understand the mechanics of what you want, but you are not giving clear answers to my questions about the interval the formula needs to handle.

So, please answer (yes or no):

1. The last date will be the last date in the data sample - yes or no?
2. The first date of the interval could be any date in the data sample - yes or no? If yes, will that parameter be stated in the workbook somewhere so that a cell can be referenced, or does it need hard-coding into the formula.

I asked if you had read post #12. Have you read it yet? You accused me of saying something that I did not say and I think you owe it to me at least to acknowledge that you got it wrong.

I did not say "Shame on you!" - that's the point. I have nothing to be ashamed of!

I have put out a call for help - someone else will step in.

Good luck.

Thank you - of course I accept your apology. I would never say that to anybody, here or in the wider world!

I have put out a call for help, as I don't seem able to get the information I need from you to help. I hope someone else will.

I will look in again later.

I have reversed the neg rep, it was clearly a misunderstanding.

That is 1 of the biggest problems/pitfalls of written communication - there is not really a good way to show intent when typing, one has to try and interpret the intention based on factors that are often outside the scope of the actual conversation.