retentionExpirationTime

A server-determined value that specifies the earliest time that the object's retention period expires. This value is in RFC 3339 format. Note 1: This field is not provided for objects with an active event-based hold, since retention expiration is unknown until the hold is removed. Note 2: This value can be provided even when temporary hold is set (so that the user can reason about policy without having to first unset the temporary hold).