Fix a build warning & change how a PPRScheduleTime describes itself

...me computes its description string.

PPRScheduleTime.m - use a DateFormatter etc., rather than a crude stringWithFormat;
PPRTestIIntialiser.m [sic] - fix the warning;
PPRScheduleTimeTests.m - added, for a crude test.
Repeat the calculation in PPRSchedueTime and check result asserting a different calendar.