It has appeared to me to be by date. That is to say it will show post made "back to" the date before. In example; If one were to do the search at midnight it would cover 48 hours. At one mintue after midnight would only yeld a search for 24 hours plus one minute.
But then I could be wrong.
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Gunslinger
TFL End of Summer Meet, August 12th & 13th, 2000