Open balance

[Raul Sousa]

I have a report with cash movements, like this:

MovDate MovValue
12-01-2007 10
13-01-2007 20
14-01-2007 5
15-02-2007 50
16-02-2007 22
17-02-2007 33
18-02-2007 15
19-02-2007 46
20-02-2007 12

What I really want is the report to show the movements from certain period
of time. It should sum the movements from previous time as opening balance.
For the period from 15-02-2007 to 20-02-2007 it should be something like:
MovDate MovValue
Opening balance 35
15-02-2007 50
16-02-2007 22
17-02-2007 33
18-02-2007 15
19-02-2007 46
20-02-2007 12

I have no idea on how to get this opening balance value.

Any suggestion is most appreciate