gml:TimeCS is a one-dimensional coordinate system containing a time axis, used to describe the temporal position of a point in the specified time units from a specified time origin. A TimeCS shall have one gml:axis property element.
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="TimeCS" type="gml:TimeCSType" substitutionGroup="gml:AbstractCoordinateSystem"><annotation><documentation>gml:TimeCS is a one-dimensional coordinate system containing a time axis, used to describe the temporal position of a point in the specified time units from a specified time origin. A TimeCS shall have one gml:axis property element.</documentation></annotation></element>