Active day count in DAX

Alberto Ferrari posted a very nice blog entry some time ago regarding how to count active days in PowerPivot (http://sqlblog.com/blogs/alberto_ferrari/archive/2011/05/12/powerpivot-counting-active-days.aspx ). His example relies on a data model that specifies 'Active From' and 'Active To' fields in order to compute the count.

I want to write a follow up entry for when such fields do not exist in the data model. If all you have is a start date reflected as a transaction recorded in your fact table, counting active days is still possible in DAX. Lets see how.