Accumulated depreciation in Excel

Some clients have had old computers for years, and it is time to write off old ones from the accounts. We have date of purchase, current year date year end, and cost. 25% reducing balance method is used. We need accumulated depreciation from date of purchase to current year end date in one formula. Trouble with Excel is that their formulas always ask for useful life of the asset, which we do not have and there seems no formula to calculate what must be a common requirement. Am I missing something, or does someone have the golden formula? Any help appreciated.