Cumulative distance between intercept points on Excel graphs

Hi there, hoping you can help with the following:

Looking at the chart below, it is possible to use an excel formula/VBA to calculate the cumulative time below a certain value of x (as indicated by the arrows I've added to the chart)?

NB The graph shows the variation in tide height (x) over time (y).

The graph looks like this:

http://i264.photobucket.com/albums/i...d/Graph1-1.jpg

The data looks a littles something like this : (extract)

http://i264.photobucket.com/albums/i...road/Data1.jpg

Really stumped on where to begin with this one; any suggestions or input would be very gratefully received.

Stuart

(Excel 2007 & Windows Vista)