#51 Add an option to approximate time

Hi,
It would be useful to have an option specifying if approximating times or not.
As an instance for a new diary entry between 9h16 to 12h54, it could be approximated to
-the nearest hour (9h00->13h00)
-the nearest half-hour (9h30->13h00)
-the nearest 15 minutes (9h15->13h00)
-the nearest 5 minutes (9h15->12h55)
-...
-...

Thanks,
Riccardo