这样做

<select id=I_Year>
</select>年
<select id=I_Mon onChange="seleDate('mon')">
</select>月
<select id=I_Date onChange="seleDate('date')">
</select>日
<script>
function genYear(sele,year,length) {
    var d = new Date();
    for(i=0;i<length;i++) {
    sele.options[i] = new Option(i+year,i+year)
    if(i+year == d.getYear())
        n = i;
    }
    sele.options.length = length;
    sele.options[n].selected = true;
}
function genMon(sele,year) {
    var d = new Date();
    for(i=0;i<12;i++) {
    sele.options[i] = new Option(i+1,i+1)
    if(i == d.getMonth())
        n = i;
    }
    sele.options.length = 12;
    sele.options[n].selected = true;
}
function genDate(sele,year,month) {
    var d = new Date(year,month,0);
    var length = d.getDate();
    d = new Date();
    for(i=0;i<=length;i++) {
    sele.options[i] = new Option(i+1,i+1)
    if(i+1 == d.getDate())
        n = i;
    }
    sele.options.length = length;
    sele.options[n].selected = true;
}
var d = new Date();
genYear(I_Year,1993,20);
genMon(I_Mon,d.getYear());
genDate(I_Date,d.getYear(),d.getMonth()+1);
function seleDate(mode) {
    switch(mode) {