Get the exact difference between 2 dates for a single NSDateComponent

Question

How can I get the exact difference (in decimal) between 2 values of NSDate.

Eg. Jan 15 2016 to Jul 15 2017 = <

Answer 1

According to NASA, there are 365.2422 days per year on average. Here, I round that up to 365.25 days per year:

let components = NSCalendar.currentCalendar().components([.Year, .Month, .Day], fromDate: fromDate, toDate: toDate, options: [])

var totalYears = Double(components.year)
totalYears += Double(components.month) / 12.0
totalYears += Double(components.day) / 365.25

Obviously, this depends on your assumptions. If you want to count of leap days between fromDate and toDate, it will be more complicated.

Some sample outputs:

From date      To date        Total Years
------------   ------------   ------------
Jan 15, 2016   Jul 15, 2017   1.5
Jan 15, 2016   Apr 14, 2016   0.25
Jan 15, 2016   Aug 15, 2017   1.5833
Jan 15, 2016   Jan 14, 2018   1.9988