See Biff's answer. Comparison on the Minute component will not work because
it will only show 5 minutes for 15:05:30 (15 hours + 5 minues +30 seconds).
and that number certainly exceeds 15 minutes.

for more information on Date and Time see
http://www.mvps.org/dmcritchie/excel/datetime.htm-
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HTH,
David McRitchie, Microsoft MVP - Excel [site changed Nov. 2001]
My Excel Pages: http://www.mvps.org/dmcritchie/excel/excel.htm