Time Calc field

In one of the tables in SQL 2000 I need to calculate the diff in # of days between several actions:

I use the following formula in the design screen of the table:

(datediff (dd, colname, getdate( ))

but I get an error.

Does this have to do with missing an = sign or not using caps? or does anyone see where the problem is.

I cannot insert the exact date as the column on which the calc needs to take place contains over 200 000 records.

Any help appreciated

Albert