Update from Linh's deconflict

Ha Linh’s deconflict removed certain parts of the codes. This commit is
to patch it back to normal.

This commit is similar to the previous further deconflict where
DateParser is now a constant in InputParser.

This commit also recaptures bugs when a user enters the command
“add”/“edit”/“done”/“delete”/“undone” without specifying the parameters.