Finding the Date Associated with a Negative Value

Stuart has a series of readings in a worksheet. In the first column he has dates associated with the readings and in the second column he has the actual readings. Stuart would like to have a formula that will return the first date at which a reading became negative. In other words, the formula should look for the first value that is negative in the second column and then return the date associated with that value. There can be multiple negative values in the second column, but he needs only the date associated with the first negative value.

There are a number of ways that this problem can be approached. All of the methods presume that the dates in column A are in ascending order and that the readings in column B are not in any type of discernable order. (In other words, the readings could bounce above and below 0 on any given date.)

Provided that you have some control over the layout of the worksheet, you could add an intermediate work column in column C, used to indicate when a value is negative. Simply place a formula like this in column C, to the right of each reading:

=IF(B1<0,A1,"")

This formula returns the date in column A if the value in B is below 0 (negative), otherwise it returns nothing. All you then need to do is look for the minimum value in column C:

=MIN(C:C)

Format the result as a date, and it represents the date at which the readings first became negative.

Another approach is to forego the use of the intermediate column and use an array formula to determine the date. Assuming the data is in the range A1:B42, you can use any of the following formulas:

=MIN(IF(B1:B42<0,A1:A42,""))
=OFFSET($A$1,MATCH(TRUE,$B$1:$B$42<0,0)-1,,,)
=INDEX(A:A,MIN(IF(B1:B42<0,ROW(B1:B42))))
=INDEX(A1:A42,MATCH(TRUE,B1:B42<0,0))
=INDIRECT("A"&MIN(IF(B1:B42<0,ROW(B1:B42))),TRUE)

Remember that these are all array formulas, so you need to enter whichever one you choose by pressing Shift+Ctrl+Enter. Format the result as a date, and it is the answer you seek.

If you prefer, you could also use a simple macro to determine the date:

Function GetFirstNegative(rngdata)
    Dim c As Variant

    For Each c In rngdata
        If c < 0 Then
            GetFirstNegative = c.Offset(0, -1)
            Exit Function
        Else
            GetFirstNegative = "All Data is Positive"
        End If
    Next
End Function

In your worksheet, you would use this user-defined function in this manner:

=GetFirstNegative(B1:B42)