Comment 1 for bug 1567640

(3:09:10 PM) mriedem: oomichi: the response value for started_at is here https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/simple_tenant_usage.py#L132
(3:09:28 PM) mriedem: which is instance.launched_at, which is an oslo.versionedobject DateTimeField
(3:10:00 PM) mriedem: which gets it's string version here https://github.com/openstack/oslo.versionedobjects/blob/master/oslo_versionedobjects/fields.py#L403
(3:10:17 PM) mriedem: and the format for that is here https://github.com/openstack/oslo.versionedobjects/blob/master/oslo_versionedobjects/_utils.py#L21