If you mean "within the past 24 hours" then (assuming the $bbuserinfo[lastpost] is set) you can check it by subtracting $bbuserinfo[lastpost] from TIMENOW and comparing the result to 86400 (seconds in 24 hours).
If you mean "on today's date" then you'd need a plugin with more code to figure that out. Basically get the day-month-year for today and the post time, and see if they're the same, or maybe get the timestamp for midnight at the start of today and see if $bbuserinfo[lastpost] is greater (and you might have to deal with timezones depending on what you mean by "today").
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