the code can be created in expression

i'm creating assets tracking data base, i have to set asset depreciation every year of asset life, i am using datediff() but it count total duration of asset, i want that starting date and ending date and computer count integer number until the ending date for example purchased asset date 12-Aug-2010 useful life of Asset is 12-Aug-2014, between both dates computer account automatically number that has been passed ..