Fix Alert-C start/end time display

[mvglasow]

Several fixes and improvements for start/end times where indicated in Alert-C messages:

• Calculations for values 232–255 were incorrect:
  • Mid/end of month were swapped
  • Month off by one for some values
• Day of month refers to current or next month, whichever occurs first in a 31-day range starting tomorrow
• Display raw value next to decoded date to facilitate debugging