A simple format Date function using Javascript prototype

formatDate.js

/*
  Based on Rick Strahl code
  http://www.west-wind.com/weblog/posts/2008/Mar/18/A-simple-formatDate-function-for-JavaScript

  Contributors:
    Clauber Stipkovic - @clauberhalic
    Mário Rinaldi - @MarioRinaldi
*/

Date.prototype.formatDate = function (format) {
    var date = this,
        day = date.getDate(),
        month = date.getMonth() + 1,
        year = date.getFullYear(),
        hours = date.getHours(),
        minutes = date.getMinutes(),
        seconds = date.getSeconds();

    if (!format) {
        format = "MM/dd/yyyy";
    }

    format = format.replace("MM", month.toString().replace(/^(\d)$/, '0$1'));

    if (format.indexOf("yyyy") > -1) {
        format = format.replace("yyyy", year.toString());
    } else if (format.indexOf("yy") > -1) {
        format = format.replace("yy", year.toString().substr(2, 2));
    }

    format = format.replace("dd", day.toString().replace(/^(\d)$/, '0$1'));

    if (format.indexOf("t") > -1) {
        if (hours > 11) {
            format = format.replace("t", "pm");
        } else {
            format = format.replace("t", "am");
        }
    }

    if (format.indexOf("HH") > -1) {
        format = format.replace("HH", hours.toString().replace(/^(\d)$/, '0$1'));
    }

    if (format.indexOf("hh") > -1) {
        if (hours > 12) {
            hours -= 12;
        }

        if (hours === 0) {
            hours = 12;
        }
        format = format.replace("hh", hours.toString().replace(/^(\d)$/, '0$1'));
    }

    if (format.indexOf("mm") > -1) {
        format = format.replace("mm", minutes.toString().replace(/^(\d)$/, '0$1'));
    }

    if (format.indexOf("ss") > -1) {
        format = format.replace("ss", seconds.toString().replace(/^(\d)$/, '0$1'));
    }

    return format;
};

support utc date formatting

Date.prototype.format = function (format, utc) {
      var date = this,
        day = utc ? date.getUTCDate() : date.getDate(),
        month = (utc ? date.getUTCMonth() : date.getMonth()) + 1,
        year = utc ? date.getUTCFullYear() : date.getFullYear(),
        hours = utc ? date.getUTCHours() : date.getHours(),
        minutes = utc ? date.getUTCMinutes() : date.getMinutes(),
        seconds = utc ? date.getUTCSeconds() : date.getSeconds();

      ....
}