和暦→西暦＋曜日算出

よくよく考えてみると、
西暦に戻すと、月日は関係ないのかも。

ということで、一気に仕上げた。

曜日まで出すよ。

$(function(){
    $('#convertButton').click(function(){
        var d = $('#date_text').val();
        $('#result').html(d.getSeireki('漢字', true));
    });
});

var weeks = ["日","月","火","水","木","金","土"];

String.prototype.getSeireki = function(pattern, day) {
    var ret = this.match(/[0-9]+/g);
    var wareki = '';
    var year = -1;
    var month = -1;
    var date = -1;
    if (ret.length == 1) {
        year = ret[0];
        wareki = this.substring(0,this.indexOf(ret[0]));
    } else if (ret.length == 2) {
        year = ret[0];
        wareki = this.substring(0,this.indexOf(ret[0]));
        month = ret[1];
    } else if (ret.length == 3) {
        year = ret[0];
        wareki = this.substring(0,this.indexOf(ret[0]));
        month = ret[1];
        date = ret[2];
    } else {
        return '';
    }
    
    var result = '';
    if ("昭和" == wareki || "S" == wareki) {
        result = parseInt(year) + 1925;
    } else if ("平成" == wareki || "H" == wareki) {
        result = parseInt(year) + 1988;
    } else if ("大正" == wareki || "T" == wareki) {
        result = parseInt(year) + 1911;
    } else if ("明治" == wareki || "M" == wareki) {
        result = parseInt(year) + 1867;
    }
    
    var divYear = '';
    var divMonth = '';
    var divDate = '';
    if (pattern == undefined || pattern == '漢字') {
        pattern = 1;
        divYear = '年';
        divMonth = '月';
        divDate = '日';
    } else if (pattern == '-') {
        pattern = 2;
        divYear = '-';
        divMonth = '-';
        divDate = '';
    } else if(pattern == '/') {
        pattern = 3;
        divYear = '/';
        divMonth = '/';
        divDate = '';
    } else {
        pattern = 3;
    }
    
    if (ret.length == 1) {
        return result + divYear;
    } else if (ret.length == 2) {
        return result + divYear + month + divMonth;
    } else if (ret.length == 3) {
        var dispDay = '';
        if (day == true) {
            var strDate = result + '/' + (month) + '/' + date;
            var d = new Date(strDate);
            var youbi = weeks[d.getDay()];
            if (pattern == 1) {
                dispDay = ' ' + youbi + '曜日';
            } else if (pattern == 2 || pattern == 3) {
                dispDay = ' (' + youbi + ')';
            }
        }
        return result + divYear + month + divMonth + date + divDate + dispDay;
    }
}