Let's assume that you want to group all your nodes by month. Do the followings:
- At admin/config/regional/date-time/formats/add, create a year-month format(e.g. Y-m).
- At admin/config/regional/date-time/types/add, create a year-month date type and link to your year-month format.
- Go to your View and open your date field. Select the year-month date type you created.
- Go to Format->Settings, set Grouping field Nr.1 to your date field.
Note: If RDF module is enabled, then check Remove tags from rendered output at Format->Settings. Otherwise, the grouping will not work.