Hi everyone.
Can you help me please because i have a problem with my project.

I have fields like date and sales quantity

date                           quantity
06/10/2010               100
10/10/2010               200
15/10/2010               250 
16/10/2010               300

I want to make a formula to calculate the running total for a specific date. For etc. when i give a parameter to bring me the sales quantity from 10/10 to 16/10 my running total  i want to show me the quantity from previus date like 06/10.
Previus Sales is 100 (from 06/10) and the current sales is 750(from 10/10 to 16/10.