Just found out that when you specify a date limit for the posts to be searched for, only the topic dates are being included. So a reply with a different date than the topic date will be left out of the records to be searched, contrary to what I think should be expected.
To fix this, change line# 298, search.asp:
strSql3 = strSql3 & " AND (T.T_DATE > '" & DateToStr(dateadd("d", -dt, strForumTimeAdjust)) & "')"
Replace it by
strSql3 = strSql3 & " AND ( (T.T_DATE > '" & DateToStr(dateadd("d", -dt, strForumTimeAdjust)) & "') "
strSql3 = strSql3 & "OR (R.R_DATE > '" & DateToStr(dateadd("d", -dt, strForumTimeAdjust)) & "') ) "