Q:

How to println NSTimeInterval in Swift? 

I want to print elapsedTime using a println or NSLog statement.  How to convert NSTimeInterval elapsedTime to String to print it? 
t_start = NSDate()
var elapsedTime = NSDate.timeIntervalSinceDate(t_start)
println("in applicationDidEnterBackground, elapsedTime = " + elapsedTime)

A:

There is a syntax error:
NSDate.timeIntervalSinceDate(t_start) should be NSDate().timeIntervalSinceDate(t_start). 
Then use String Interpolation in the println statement. 
Here is a corrected version:
let t_start = NSDate()
let elapsedTime = NSDate().timeIntervalSinceDate(t_start)
println("in applicationDidEnterBackground, elapsedTime =\(elapsedTime)")

Output:  

"in applicationDidEnterBackground, elapsedTime =0.00790297985076904"

With fewer decimal points use String format:
println(String(format: "in applicationDidEnterBackground, elapsedTime =: %.3f sec", elapsedTime))

Output  

in applicationDidEnterBackground, elapsedTime =: 0.008 sec

