Calculate interval since last positive occurance

Hello,

I am trying to calculate the interval between only positive entries on on dynamic table. A sample data set is listed below and attached as a spreadsheet. The interval calculation is in the final column. If the current line has a positive response, the formula will return the number of days since the last positive. If the current entry is negative, a null value is returned. The average interval is calculated elsewhere for planning reasons using: =AVERAGE(C:C)


Date Ate Interval
1/5/17 yes 7
1/12/17 No -
1/19/17 yes 14
1/26/17 No -
2/4/17 yes 16
2/11/17 no -
2/17/17 No -
2/18/17 yes 14

Average Use Interval: 13