|Your value in B15 is a formula instead of a value.|
Try entering your dates column with simply the data as 2006,2,17 and then your VLOOKUP will find it as a text string.
Otherwise, I am using Excel 97 and find your date format as not a vaild format for a date. It correctly sees it as a text string. (Perhaps in later versions of Excel your format is a valid date format?)
If you use a date format such as 2/17/2006 and use that as the lookup value then the function will convert your dates into numeric values and find it by the same value. eg 2006,2,17 as 2/17/2006 converts to 38765.