Firebird 3.0

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create table Events (d date, EventsCount integer)
insert into events values(date '2022-04-01', 15)
1 rows affected

insert into events values(date '2022-09-30', 10)
1 rows affected

insert into events values(date '2022-10-01', 1)
1 rows affected

insert into events values(date '2022-10-01', 1)
1 rows affected

create procedure DaysSinceNthEvent(n integer) returns (days integer) as
  declare c integer;
  declare dd date;
  declare total integer;
begin
  total = 0;
  for select d, EventsCount from Events order by d desc into dd, c do
    begin
      total = total + c;
      if (total >= n) then
        begin
          days = current_date - dd;
          suspend;
          exit;
        end
    end
end
select * from DaysSinceNthEvent(3)


DAYS


31