Since ISO 8601 represents date and time, not an interval, so I assume 500 ms from Thursday, January 1, 1970 (exactly the same as the unix timestamp).
1970-01-01T00: 00: 00.5Z
Update: Yes "PT0.5S" is the correct interpretation according to the documentation you are referring to. Apparently my previous answer missed some of the time in the documentation or it can be added later. Thanks @ strife25
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