conditional sum derived from a date interval

[tghcogo]

Hi,

I'm trying to sum each months visa purchases. The data is in three columns: date,value and payment type.

I can't get this formula to work:-

{=SUM(IF(AND(Purchases!C6:C999>DATE(2009,12,28),Purchases!C6:C999<DATE(2010,1,29)),IF(Purchases!K6:K999="Visa",Purchases!H3:H999)))}


It's returning a value of 0 (and if that were true I'd be delighted!)

Thanks in advance.