Took some head scratching from me too, it's been a while. It was a code modification to the original forum software (a patch package, I think, which I'd adapted).
It's in Sources/Recent.php
'bugun' => intval(time()-2*24*60*60), // another set down below too - search bugun
'yarin' => intval(time()),
Changed it to:
'bugun' => intval(time()-4*24*60*60), // another set down below too - search bugun
'yarin' => intval(time()),
I remember writing the comment and sure enough there is another one of those calculations below, no idea why, just modified them both.
So that's 2 days to 4 days, see how that feels for a bit
I'm guessing the original coder was Turkish - bugun = today, yarin = tomorrow. (The original code only showed the last 24 hours, if I remember rightly, so "yarin" isn't technically correct as a variable name any more...)