if the values are in row 2 and dates in row 1, try:

=LOOKUP(1,1/(2:2=500),1:1)


"CC" wrote:

I am trying to search a row of numbers for the first occurence of a number
=500 and return the date at which the number is found, where the dates are
the column headings in the spreadsheet. I can use LOOKUP to find the date
where the value = 500 but I need to find when it becomes =500.