For DATEDIFF() function
Try this:
UPDATE A
SET A.monthDiff = DATEDIFF(mm, CONVERT(DATE, A.orderDate, 112), B.lastOrderDate),
A.dayDiff = DATEDIFF(dd, CONVERT(DATE, A.orderDate, 112), B.lastOrderDate)
FROM tableA A, (SELECT MAX(CONVERT(DATE, orderDate, 112)) lastOrderDate FROM tableA) B
Check the SQL FIDDLE DEMO
OUTPUT
| ID | ORDERDATE | MONTHDIFF | DAYDIFF |
|----|-----------|-----------|---------|
| 1 | 20130105 | 9 | 283 |
| 2 | 20130205 | 8 | 252 |
| 3 | 20130305 | 7 | 224 |
| 4 | 20130909 | 1 | 36 |
| 5 | 20131001 | 0 | 14 |
| 6 | 20131015 | 0 | 0 |