Rounding Time

I'm trying to round a time to the nearest hour on a 24 hr clock. I'm currently trying something like:

    TO_CHAR(L.LOGIN_DATE,'fmHH24')+(TRUNC(TO_CHAR (L.LOGIN_DATE,'fmMI')/30)) || ':00' as "Rounded"

But the problem with this is 11:31pm rounds up to 24:00 but 1:29am rounds to 0:00. I can't have both 24:00 and 0:00. I'd like 11:31pm to round to 0:00. Any ideas?

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