Open Radar

 
Summary:
If your device is set to use 24hr time (instead of AM/PM), the NSDateIntervalFormatter will still show times in AM/PM format. NSDateFormatter will correctly format using the 24hr time value.

Sample App:
https://github.com/jffmrk/NSDateIntervalFormatterBug

Steps to Reproduce:
1 - Device is in en_US locale
2 - Go to Settings -> Date & Time
3 - Turn on "24-Hour Time" switch
4 - Format a time interval using NSDateIntervalFormatter

Example code:

// Setup dates
let now = NSDate()
let next = NSDate(timeInterval: 14400)

// This formats 24-hr preferences correctly
let dateFormatter = NSDateFormatter()
dateFormatter.dateStyle = .MediumStyle
dateFormatter.timeStyle = .LongStyle
let str = dateFormatter.stringFromDate(now)
NSLog("Date: \(str)")
        
// This ignores the 24-hr preference toggle
let dateIntervalFormatter = NSDateIntervalFormatter()
dateIntervalFormatter.dateStyle = .MediumStyle
dateIntervalFormatter.timeStyle = .LongStyle
let strInterval = dateIntervalFormatter.stringFromDate(now, toDate: next)
NSLog("DateInterval: \(strInterval)")

Expected Results:
NSDateIntervalFormatter should format the times using the 24hr format or 12hr format depending on device setting.

Actual Results:
NSDateIntervalFormatter uses 12hr time format

Version:
iOS 9.2.1 (13D15)