collapse time_timestamp calculation

while the equation is a little lengthier, it's more apparent what's
going on, which it exposes some oddities in the calculation (fixed in
next commit)

Change-Id: Ibcb4005f987972961d406f5f1d9d0fd875d39f78
Reviewed-on: https://gerrit.instructure.com/31596
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>