crd:velTimeUnitType 

Velocity units are a posUnitType / velTimeType; the latter may be second, day, hour, year (a or yr), century, but not empty

Simple Type Information

Used By

Attributes crd:Velocity1D/@vel_time_unit, crd:Velocity2D/@vel_time_unit, crd:Velocity3D/@vel_time_unit, crd:astroCoordsType/crd:Redshift/@vel_time_unit, stc:redshiftIntervalType/@vel_time_unit, stc:velocityIntervalType/@vel_time_unit, stc:velocitySphereType/@vel_time_unit

Source 

<xs:simpleType name="velTimeUnitType">
  <xs:annotation>
    <xs:documentation>Velocity units are a posUnitType / velTimeType; the latter may be second, day, hour, year (a or yr), century, but not empty</xs:documentation>
  </xs:annotation>
  <xs:restriction base="crd:unitType">
    <xs:enumeration value="s"/>
    <xs:enumeration value="h"/>
    <xs:enumeration value="d"/>
    <xs:enumeration value="a"/>
    <xs:enumeration value="yr"/>
    <xs:enumeration value="cy"/>
  </xs:restriction>
</xs:simpleType>