Calculating an inventory balance at a "point in time"

All,

I have an inventory transaction table that houses all additions and
subtractions of a particular part #. The inventory balance of the
part # at any point in time is simply the sum of all the additions and
subtractions.

I need to create a report that can display the inventory transactions
and balances at a specific point in time.

For example, my transaction table has 4 transactions from 7/1 –
8/15.

Transaction 1: on 7/1 added 100 units of part #1
Transaction 2: on 8/1 added 100 units of part #1
Transaction 3: on 8/5 subtracted 15 units of part #1
Transaction 4: on 8/15 subtracted 75 units of part #1

So my inventory balance as of 8/15 is 110 units of part #1.

Now I wish to run a report that uses some date parameters that require
some “point in time” calculations. For example; I would like to run a
report that says for each transaction listed, show me the balance of
my inventory from 8/4 – 8/15. Each time inventory changed calculate
the new amount of inventory and the # of days that the inventory
balance was that amount.


OUTPUT
Starting balance on 8/4 = 200 units of part #1

On 8/5: 15 units of part #1 were subtracted leaving a balance of 185
units for 1 day (from 8/4 - 8/5 there were 100 units of Part #1)

On 8/15: 75 units of part #1 were subtracted leaving a balance of 110
units for 10 days (From 8/5 - 8/15 (10 days) there were 85 units of
Part #1 (100 – 15))

QUESTION
I am having a hard time figuring out how to get my report to display
the starting inventory balance for the start date of the report. This
date needs to be a parameter value as users may wish to choose any
date and I would like to prompt them for the value they wish to use.

Basically the other line item calculations are easy enough to figure
out but I am not sure how to make the first line item on the report be
the starting date of the report (parameter #1) along with the
inventory at that point in time.

Any help would be much appreciated.
Thanks