I need to subtract the value of a column based on the filter date and the last value

[kfloresc]

I'm new to qlik sense and am having trouble with a simple subtraction with a restricted date filter. Any help would be appreciated 

I am creating a straight table where I need to find the difference between a column's current value based on a selected filter and the value of that column from the last entry.

I need to have the filter date within the sheet where users select a date. I have column X (cost), I need to add a column Y (current cost based on selected filter date - the previous value, which is associated with another date)