Is there way to shows users online in past 24 hours?
Go in your Admin CP -> Configuration -> Settings -> Who's Online -> Cut-off Time (mins) -> put "1440"
without the ""
( 1440 minutes = 24 hours )
Alternatively, go to the "Who's online"-page (bottom of the index-page, under Board Statistics, click the link "[complete list]") and hit "Who was online today".