check assignments API date times with api regex

refs CNVS-10087

test plan
 - specs should pass

Change-Id: Ia28a3ec65c24d2e70a5073b5da2caab8b282da9f
Reviewed-on: https://gerrit.instructure.com/33115
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>