fix bad date math in gradebook dialog

fixes CNVS-12386

test plan:
- setup some assignment with submissions in different combinations of:
  - late / on-time
  - single submission / multiple submissions
- make sure that GB2 correctly differentiates between late and on time in the
  submission details dialog (that pops up when you click in the top right
  corner of a cell)
  - it will be text for a single submission and dropdown for multiple

Change-Id: Ib5f62dd95b073461cfcbea85b1ed8e0805f2a57f
Reviewed-on: https://gerrit.instructure.com/33263
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>