Sync timeutils from oslo-incubator

Sync the following commit:

ae931a9 Avoid TypeError in is_older_than, is_newer_than

Change-Id: I963c79599fe6731dd84c90ee3f3fbfb02505e045
Partially-implements: blueprint periodic-tasks-to-slave
Related-Bug: #1259383
Merge "Sync timeutils from oslo-incubator"