Count number of empty cells since last non-empty cell

I used this method to calculate days since last time exercise was done. This is an array formula entered in a row inserted at row 2. Enter in B2 and fill across.

Formula:

=TODAY()-INDEX($A4:$A1200,MAX(IF(B4:B1200="x",ROW(B4:B1200)))-3)

Enter with Ctrl + Shift + Enter

This will allow the table to be extended down to row 1200 to give you a little over 3 years of dates to be entered without doing anything other than filling in the data.