js中日期的加减法

var dateFormat = function () {
  var  token = /d{1,4}|m{1,4}|yy("[^"]*"|'[^']*'/g,
    timezone = /\b("0" + val;
      return val;
    };
 
  // Regexes and supporting functions are cached through closure
  return function (date, mask, utc) {
    var dF = dateFormat;
 
    // You can't provide utc if you skip other args (use the "UTC:" mask prefix)
    if (arguments.length == 1 && Object.prototype.toString.call(date) == "[object String]" && !/\d/.test(date)) {
      mask = date;
      date = undefined;
    }
 
    // Passing date through Date applies Date.parse, if necessary
    date = date "invalid date");
 
    mask = String(dF.masks[mask] || mask || dF.masks["default"]);
 
    // Allow setting the utc argument via the mask
    if (mask.slice(0, 4) == "UTC:") {
      mask = mask.slice(4);
      utc = true;
    }
 
    var  _ = utc "getUTC" : "get",
      d = date[_ + "Date"](),
      D = date[_ + "Day"](),
      m = date[_ + "Month"](),
      y = date[_ + "FullYear"](),
      H = date[_ + "Hours"](),
      M = date[_ + "Minutes"](),
      s = date[_ + "Seconds"](),
      L = date[_ + "Milliseconds"](),
      o = utc "a" : "p",
        tt:  H < 12 "am" : "pm",
        T:  H < 12 "A" : "P",
        TT:  H < 12 "AM" : "PM",
        Z:  utc "UTC" : (String(date).match(timezone) || [""]).pop().replace(timezoneClip, ""),
        o:  (o > 0 "-" : "+") + pad(Math.floor(Math.abs(o) / 60) * 100 + Math.abs(o) % 60, 4),
        S:  ["th", "st", "nd", "rd"][d % 10 > 3 "default":   "ddd mmm dd yyyy HH:MM:ss",
  shortDate:   "m/d/yy",
  mediumDate:   "mmm d, yyyy",
  longDate:    "mmmm d, yyyy",
  fullDate:    "dddd, mmmm d, yyyy",
  shortTime:   "h:MM TT",
  mediumTime:   "h:MM:ss TT",
  longTime:    "h:MM:ss TT Z",
  isoDate:    "yyyy-mm-dd",
  isoTime:    "HH:MM:ss",
  isoDateTime:  "yyyy-mm-dd'T'HH:MM:ss",
  isoUtcDateTime: "UTC:yyyy-mm-dd'T'HH:MM:ss'Z'"
};
 
// Internationalization strings
dateFormat.i18n = {
  dayNames: [
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat",
    "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"
  ],
  monthNames: [
    "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
    "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"
  ]
};
 
// For convenience...
Date.prototype.format = function (mask, utc) {
  return dateFormat(this, mask, utc);
};

var nowDate = new Date();
//加法操作
var ysDate = new Date(nowDate.getTime()-30*24*60*60*1000).format('yyyy-mm-dd');
//减法操作
var yeDate = new Date(nowDate.getTime()-24*60*60*1000).format('yyyy-m-d');