>>>Hi,
>>>
>>>How do you SUM number of days difference between two DateTime type fields as following:
>>>
>>>

>>>select SUM( DATETIME1 - DATETIME2) AS NUMDAYS
>>>

>>

>>SELECT SUM(DATEDIFF(dd, Date1, Date2)) AS NumDays
>>

>
>Borislav,
>
>I am trying to understand the DATEDIFF() function better. Do I understand correctly that the 1st parameter "dd" really has no literal meaning? That is, you can name this parameter anything and it won't change the result?

It has, it means days :-)
As Naomi said it means we wand to get difference in days. Check BOL to see the other values of the first parameter.