In essence each share gets a "score". The score is highest at the round end, and earlier shares get lower scores. The older the share the lower the score. And that progression is not linear but exponential, so score goes down to almost zero very quickly. From observations I think the cut-off is about an hour - for shares older than an hour you get absolutely no score.
Not argue, you are correct. Just giving information about cut-off also being based on observation:
Cutoff not exactly being one hour or based on time. Cutoff depending on total Slush pool hash rate and when score being renormalized after hitting 14 significant digits. Renormalize used be happening just over one hour when hash rate was about 9Tb. With current hash rate renormalize happening about each 52 minutes. Am thnking is why some rewards suddenly crash to .00000007 if being no shares submitted exactly between renormalize and round ending. Few shares at normal rate in seconds after renormalize taking reward back to full for next scoring period, but scores in round also decrease very quickly by not submitting shares at constant rate all time.