Apply restriction to time attribute in XML Schema

I want to apply a specific restriction to an XML Schema (on which I have very little experience).

I have an attribute of xsd:time type:

<xsd:attribute name="hour" type="xsd:time" use="required"/>

What I want to to is apply a restriction so that the XML will be valid only on half-hour time intervals. For example, 10:00, 12:30, 15:30, 20:00 would be valid values for the hour attribute but 10:45, 11:12, 15:34 etc would not.

How can I achieve this? My search did not give out something useful.

Thank you in advance.