for example in the sample sheet if I add dates and data going forward into 2016 and move the system date forward to a date past 31st December, instead of continuing to count the accumulated sales number it reverts to zero.

The sales year would run from 1st Sept 2015 to 31st august 2016, so the formula needs to count the accumulated sales to todays date whatever that may be.
I want to be able to see the sales position across a number of years at the current date.
Does that make anymore sense ?