Code Cleanup

- Factorize code for parsing date elements.
- Better order of blocks for parsing date elements.
- Add corresponding minimal-digit date parsing method for reading seconds,
  minutes and week.
- Update documentation and test.

ChangeLog

@@ -7,6 +7,7 @@ Features
  + Stop consider new tasks after quitting a bulk change.
  + Removed deprecated 'fg:' and 'bg:' attributes.
  + The 'diagnostics' command now reports libuuid details.
+ + New characters for parsing and formating dates ('n', 's' and 'v').
 
 Bugs
  + Fixed bug #1043, where aliases were not recognized by bash autocompletion.