Update field with value of today() - date field.

Gurus,

I am associating an opportunity with the CO3 object. I have hardcoded the associated date field with today().

now i need to calculate the number of days since the date the opportunity was associated until the opportunity is in closed sales stage or if a new opportunity is associated to the same record.I have a when modified record saved workflow that calculates the value for the no of  days associated  field as today() - date associated.

this gives me the correct value. for example if the date associated is 11/16/2012 and i modify the record today 11/19/2012, it gives the value as 2 days depending on the time the record was modified.