SQL SERVER - How get difference between dates (here in days)

Here is example difference between two dates in days (first parameter), but you can use for example hour or month, etc.

select salesid, linenum, itemid, qtyordered, createddatetime, confirmeddlv, 
datediff( day, createddatetime, confirmeddlv ) as diffdays 
from salesline
where
confirmeddlv > '2017/12/01' and
datediff( day, createddatetime, confirmeddlv )  <= 7

Output:

salesid              createddatetime         confirmeddlv            diffdays
-------------------- ----------------------- ----------------------- -----------
PO171475             2017-12-08 10:51:45.000 2017-12-10 00:00:00.000 2
PO171475             2017-12-08 10:52:03.000 2017-12-10 00:00:00.000 2
PO171675             2017-11-27 19:58:26.000 2017-12-04 00:00:00.000 7
...