CRM 2015 Update 1 introduces some new calculated field formulas. One of the most exciting additions is the DiffinDays function. This function calculates the difference between two date fields in days.
So if you want to see the length of time that a record, like a case, has been open, you now can do it without any custom code. For more information, read Steve Foster’s blog.
How would you actually calculate how long a case has been open. You have the created date on the case but the current date is not a field you have access to.
You can use Now in your formula for current date.