<script type="text/javascript"><!--
jyu = new Array("庚","辛","壬","癸","甲","乙","丙","丁","戊","己");  //
jyuni = new Array("申","酉","戌","亥","子","丑","寅","卯","辰","巳","午","未");

gengou = new Array();
gengou[0]=["M","明治",-3197178000000];
gengou[1]=["T","大正",-1812186000000];
gengou[2]=["S","昭和",-1357635600000];
gengou[3]=["H","平成",600102000000];

Date.prototype.wareki = function(s) {
dd=this;
yy = dd.getFullYear();

y10=yy%10;
eto10=jyu[y10];
y12=yy%12;
eto12=jyuni[y12];

dd_time =  dd.getTime();
for (i = 0; i < gengou.length; i++) {
  if(gengou[i][2] > dd_time){
     if(i){
        gn=new Date(gengou[i-1][2]).getFullYear();
       if(s==2){
        ge=yy-gn+1;
        yy=gengou[i-1][0]+ge;
       }else{
        ge=(!(yy-gn))?"元":(yy-gn+1);
        yy=gengou[i-1][1]+ge;
       }
     }
    break;
  }
}
if(gengou[gengou.length-1][2] < dd_time){
        gn=new Date(gengou[gengou.length-1][2]).getFullYear();
       if(s==2){
        ge=yy-gn+1;
        yy=gengou[i-1][0]+ge;
       }else{
        ge=(!(yy-gn))?"元":(yy-gn+1);
        yy=gengou[i-1][1]+ge;
       }
 }
mm = dd.getMonth() + 1;
dd = dd.getDate();
reki = yy+"年"+mm+"月"+dd+"日";
if(s==1)reki += "（"+eto10+eto12+"）";
return reki;
}
/*himajin.moo.jp*/
// --></script>

n	動作(明治以降)
0	平成**年**月**日
1	平成**年**月**日(干支)
2	H**年**月**日

引数0alert(new Date(1926,11,25).wareki());
引数1alert(new Date().wareki(1));
引数2alert(new Date().wareki(2));