Fixed timer format when system uses 12-hour clock

There is currently an issue where if the device has 12-hour clock set in the system settings, the time format for the timer label will be incorrect.

For example, instead of displaying "00:02:00" it will display "12:02:00 am".

See the following discussions on Stack Overflow:
http://stackoverflow.com/questions/11062280/nsdateformatter-in-12-hour-mode
http://stackoverflow.com/questions/2135267/nsdateformatter-with-24-hour-times