Average of range of dates with hidden / filtered rows

Hi Excel Experts,

I need urgent help with the following problem:

I have three rows (each of which stands for one document that undergoes a particular review cycle) Per row (ie document), I know Date B and Date A (with Date B being later in time than Date A). Now I want to calculate the average duration (in days) across all rows, i.e. I need to look at what is the duration in days per row; then sum this up; then divide by the count of rows.

So far, I have identified following solutions to calculate the average:
=SUM(($J$5:$J$8)-($D$5:$D$8))/COUNT($D$5:$D$8)
and then enter as an array.
[J5:J8 and D5:D8 are the ranges in which Dates B respectively A can be found]
This formula works out well (albeit it may not be very elegant).

However, there is one complication: I have hidden and filtered some rows. If I use the formula above, this shows all rows, not just the filtered ones and no longer gives the correct average.

I have tried to adapt the above formula in SUBTOTAL, i.e.
=SUBTOTAL(101, (($J$5:$J$8)-($D$5:$D$8))), entering as array
or =SUBTOTAL (101, SUM(($J$5:$J$8)-($D$5:$D$8)))
and also =SUBTOTAL(109,(($J$5:$J$8)-($D$5:$D$8)))/SUBTOTAL(102,($D$5:$D$8)), entering as array

but none of it works.

Can anybody help?

Thank you!!

Could you use a helper column with this formula in row 5 filled down to row 8..
=J5-D5
Then this formula for the average, assuming column K has the above formula...
=SUBTOTAL(109,K5:K8)/SUBTOTAL(102,K5:K8)
Then the average is based on the visible rows only.

Dear Martin, dear beaunydal,

thanks a lot for your quick replies! Actually both solutions work perfectly, I will go with Martin's one as it does not need a helper column there.

Thank you so much again!