I have written this kind of tool to scan Apache log files. The actual problem statement was "return the logs starting at XX:XX:XX and running until YY:YY:YY".

I used a binary search to position myself at the last entry with a time-stamp < XX:XX:XX and then started a sequential read until I reached a timestamp > YY:YY:YY. I used seek() and tell() to get/set my position in the file; calculated the mid-point of my range and then looped through until I got with in range. The only really tricky part was remembering to do two reads at the mid-point to get to a full record with a time-stamp in it (tell() and seek() work on bytes not records).

Pseudo-code:

$start=0, $stop=length of file(array)

# Now check the entry at $mid for my start/stop condition

$again=1

Loop-while($again=1)
    $again=0

    $mid=int(($stop - $start)/2)

    if (array[$mid] < $beg)
        $start = $mid
        $again=1
    
    if (array[$stop] > $end)
        $stop = $mid
        $again=1

End Loop
[download]

When you fall out of the loop, $start will be the first entry <= $beg and $stop will be the first >= $end.