reset when start new element in column

I'm trying to count the difference of dates reseting for each new user:

Please Login or Register  to view this content.

In the above example I have the userid and Date columns and have to create the Days column as the difference of the first day with each of the following days for each userid, so I have to reset the difference when the userid changes therefore I need to take as first day the one that corresponds to the new user as in the example

NON VBA solution

C2 =IF(A2<>A1,0,$B2-INDEX($A$2:$B$41,MATCH($A2;$A$1:$A$41,0),2)) and drag down

Attachment added.

Hi ana,

If you had 2016 Excel which has a new function called MinIFS you would put in C2 this formula:

=B2-MINIFS(B:B,A:A,A2)

and pull it down for the whole column.

There is also an Array answer for this problem.

=B2-MIN(IF($A$1:$A$41=A2,$B$1:$B$41,""))

You need to enter the above in C2 and then enter it using a Control+Shift+Enter (CSE) keystroke combination.
You will see "{ }" around your formula if done correctly.

CSE Days since minimum by ID.xlsx

You could also enter this CSE formula:
=B2-MIN(IF(A:A=A2,B:B,""))

which will allow you to enter more rows and it will calculate correctly but take longer.