When a Compute Image is deprecated a DeprecationStatus must be provided in the request body. Part of deprecation status are 3 timestamps (deprecated, obsolete, deleted) that are documented to be:

An optional RFC3339 timestamp on or after which the deprecation state of this resource will be changed to DEPRECATED/OBSOLETE/DELETED.

However, the service does no validation and any string can be submitted for such fields. In fact older images (e.g. backports-debian-7-wheezy-v20131127) have non-RFC3339 timestamp values:

"deprecated": {
  "state": "DEPRECATED",
  "replacement": "https://content.googleapis.com/compute/v1/projects/debian-cloud/global/images/backports-debian-7-wheezy-v20131127",
  "deprecated": "1970-01-01",
  "obsolete": "1970-01-02",
  "deleted": "1970-01-03"
}