Large data set gives unexplained outlier

On this sheet, there is a data point on "March 6th 2017" that is clearly not in the X column, and is biasing the X-axis badly. Why is it there, and how do I get rid of it? Or find it, even (since the most recent date is 29 October 2015).

I understand the graph looks strange. This is because there are duplicate date values, but formula values in the adjacent column, so it is read as "zero".

unknown.xlsx