Subtract 1 from the formatted end_at date.

The ranges on the API are inclusive date ranges so there is a mismatch
with the date time ranges stored in the database.

Date time range (midnight Sunday to midnight following Sunday)
2012-12-09T00:00:00+00:00 - 2012-12-16T00:00:00+00:00

Date range (Sunday to Saturday)
2012-12-09 - 2012-12-15