A gml:TemporalDatum defines the origin of a Temporal Reference System. This type omits the "anchorDefinition" and "realizationEpoch" elements and adds the "origin" element with the dateTime type.
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
Source
<element name="TemporalDatum" type="gml:TemporalDatumType" substitutionGroup="gml:AbstractDatum"><annotation><documentation>A gml:TemporalDatum defines the origin of a Temporal Reference System. This type omits the "anchorDefinition" and "realizationEpoch" elements and adds the "origin" element with the dateTime type.</documentation></annotation></element>