Try wrapping this with the CONTAINS formula with an IIF so that a 1 is returned for a match and a 0 for no match. Then you can use the results in your view as a SUM to show the number of matches in each time period.
Hope this helps!
Ryan's suggestion looks like a winner.
I just have one additional question though.
Can the string exist in the field more than once? And if so, do you need to count how many time it's there?
If you don't care about duplicate occurrences in the string, you're done. If you need to count multiple occurrences, you'll need something more. And let's cross that bridge only if you need it.
Yup that worked. Like I said - something obvious.
I'm pretty happy to just have a count of the number of rows contain that string, really. So no real need to worry about duplicates for now. Thanks!