# How to find latest date in a range based on another columns data

I have an excel spreadsheet.  Col A are dates,  Column N is a count of activity.  3 counts must be within in 90 days.  I am trying to make a formula to look at a range of rows to find the latest date based on the count of activity to determine when my activity will expire.  This is for a pilot who needs to make 3 take of and landings every 90 days.  So in the example below the result would be 10/30+90 days within the last 3 take off and landings - 2 on 10/30 and 1 from 11/2 - The pilot would be current to 1/28/18

Ex. A = Current until 10/30 + 90 days.  Answer - 1/28/18

11/02/17   1
10/30/17   2
07/20/17   1

Ex. B = Current until 11/2 + 90 days Answer - 1/31/18

11/02/17   3
10/30/17   2
09/20/17   1

Ex. C = Current until 9/20 + 90 days Answer - 12/1/17

11/02/17   1
10/30/17   1
09/20/17   7

I can't figure out a formula to calc this based on the dates in a range and the counts in another range.
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Commented:
Hi,

pls try as an array formula (Ctrl+Shift+Enter)
``````=INDEX(A2:A4,SUM(IF(SUBTOTAL(9,OFFSET(B2,,,ROW(B2:B4)-ROW(B2)+1))<3,1,0))+1)+90
``````
Regards
EE20171103.xlsx

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Author Commented:
Thank you, for all the examples the value is always 10/18 which a valid date for these test dates
Commented:
What do you refer to?
Commented:
in my file
these are the results

C2                      G2                      K2
01/31/2018      12/19/2017      01/28/2018
Author Commented:
Hmm odd, would you be able to attach your test Excel file?  Must be something I'm doing wrong.
Commented:
it's already in  message #a42353410 above
Author Commented:
Sorry thanks, let me test with my spread sheet
Author Commented:
Found the issue, working great thanks for your help, an amazing formula!
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