How can I modify the algorithm of 'Nearest in time'

[Miao Jiang]

Here is some prices in 'Price Editor'
Date Price  Source
08/28/2009 13.68 Finance::Quote
08/27/2009 10.00 user
08/26/2009  9.00 user
08/25/2009  8.00 user
08/24/2009  7.00 user
08/23/2009  6.00 user

and The asserts Over Time shows
08/24/2009    18,400.00 ( 8.00)
08/25/2009    20,700.00 ( 9.00)   
08/26/2009    23,000.00 (10.00) oops, it use the price of date 8/27
08/27/2009    23,000.00 (10.00)
08/28/2009    31,464.00 (13.68)   
08/29/2009    31,464.00 (13.68)

It seems the date before 8-27, The chart is use price of next day.
So I want to check or modify the algorithm of 'get the price' in 
'Nearset in time'
But I have no idea where the code is. Can anyone help me?

Thanks
Miao