OVER FUNCTION

Hi all, 

I need some help. I've been spending the last few hours trying to resolve an issue I'm having when using the OVER function in SQL. 

I'm trying to create a rolling moving average (in my case 30 days) however I don't want SQL to look back and calculate it based on the 30 previous rows. In my case, I wish it does it based on my date field. 

For example, as per the table below, I wish to know the maximum size on each date when compared to the previous 30 days. In my first entry (7/17/2013), the answer would be 2161. In the second (7/24/2013) entry it would be 2161 again. In my third (11/27/2013), fourth (12/04/2013) and fifth (12/12/2013) entry the answer would be 1414. Basicaly, I need SQL to go back in time on every date, look over the previous 30 days and determine the MAX value each time. 

The Query I use the following:

SELECT Date, Product, MAX([uANCT.P30D]) OVER (PARTITION BY Product ORDER BY Date, Product ROWS 30 PRECEDING

However, it looks for rows (30 preceding) and I need it to work for the 30 PRECEDING days

Can someone please help me. It's really appreciated!

Cheers,

Alex

DateSize