Move all datetime handling into a shared core module #1024
Description
See #1021 (comment)
We have (essentially) a novel approach to datetime in every sub-package, so we should get unified.
- From https://github.com/GoogleCloudPlatform/gcloud-python/pull/1044/files#r36545736: Hmm, maybe
_EPOCHshould be moved up togcloud._helpers.