how to trunc date column to half an hour

Hi All,
Is there a way of displaying date columns, truncated to half an hours.

For example: select trunc(sysdate,'HH') from dual;

Regards,
Derya.

FYI. We are usig this sort of functions in group by expressions.

PS. Ghassan thank you for your answer regarding local/global index es in partitions.

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