PHP碰到一个怪问题，做一个排班表的时候，循环显示时间的时候到第五个就多加了一秒！！！！怪

<?php
        $dtnext = "";
        $dt = "";
        $nexttime = "";
        $tm = strtotime($strStarttime);
        $tm = $tm + $strStarttime * 60;
        //echo $tm."<br>\n";
        //$dt = date('H:i', $tm);
            
        for($j = 1; $j <= $strInquiryCount; $j++) {
            if( $j == 1){
                $dt = date('H:i', $tm);
                $nexttime = $dt;
                echo "<tr width=\"238\">\n\r<td width=\"238\">".$strStarttimeTmp."-".$dt."</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r</tr>";
            }else{
                echo "<br>".$j.":".$nexttime."<br>";
                $tm = strtotime($nexttime);
                $tm = $tm + $nexttime * 60;
                //echo $tm."<br>\n";
                $dtnext = date('H:i', $tm);
                echo $j."--".$nexttime."++".$tm."++".$dtnext."<br>\n";
                echo "<tr width=\"238\">\n\r<td width=\"238\">".$nexttime."-".$dtnext."</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r<td width=\"238\">预约</td>\n\r</tr>";
                $nexttime = $dtnext;
                $tm = "";
            }
                    }
 ?>