性能问题在一段时间内找到工作日

I have some methods that return the number of weekdays between two given dates. Since calling these methods become very expensive to call when the two dates lie years apart, I'm wondering how these methods could be refactored in a more efficient way. The returned result is correct but I feel that the iphone processor is struggling to keep up and consequently freezes up the application when I would call these methods over a period of say 10years. Any suggestions ?I have some methods that return the number of w