Use the correct "completed" time in Harbormaster display UI

Summary: Fixes T12883. The task seems correct to me and I think this is a copy/paste mistake that probably blames to me.

Test Plan: Fiddled these numbers, viewed a build in Harbormaster, saw the adjusted time.

Reviewers: chad, amckinley

Reviewed By: chad

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12883

Differential Revision: https://secure.phabricator.com/D18177