Cast old duration values for unit tests to float in DifferentialController

Summary: Fixes T10549.

Test Plan: N/A

Reviewers: #blessed_reviewers, avivey, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T10549

Differential Revision: https://secure.phabricator.com/D15452