I have a list of 6000 events and the date each occurred.
For each date, I need to count the number of events since 90 days prior.
How can I count backwards until I reach the first occurrence of a date that
is more than 90 days earlier (and then stop)?

Exit Date 90 Days Ago # Exits in Between
1-Jan-1978 3-Oct-1977
12-Apr-78 12-Jan-1978 0
12-May-78 11-Feb-1978 1
1-Jul-78 2-Apr-1978 2
1-Jul-78 2-Apr-1978 2
20-Jul-78 21-Apr-1978 3
2-Aug-78 4-May-1978 4
What formula can I use to calculate the last column?
Many thanks.