Print NominalDiffTime as hours, minutes and seconds

You can print a DiffTime -- which actually represents a duration, and is likely the type you should be using -- by going through TimeOfDay. Getting your hands on a DiffTime correctly is actually a little bit tricky; you'll need:

• A leap second table. type documentation, a page with a leap second table; you'll want to have a way for your program to read this in at runtime, as these change every few months.
• From there, you can use utcToTAITime to convert to AbsoluteTime, and
• diffAbsoluteTime to get a DiffTime, and
• timeToTimeOfDay to have the library do your divmod's for you, and finally
• formatTime to print this.