determineDate(date, refDate, defaultDate) Usage: determineDate('+1y') = now + 1 year determineDate('+1d', new Date(2012, 1, 1)) == 2012-01-02

gistfile1.js

function determineDate(date, refDate, defaultDate) {
    var getDaysInMonth = function (year, month) {
      var date = new Date(year, month, 32);
      date.setHours(date.getHours() > 12 ? date.getHours() + 2 : 0);
      return 32 - date.getDate();
    },
    offsetNumeric = function (offset, date) {
        date.setDate(date.getDate() + offset);
        return date;
    },
    offsetString = function (offset, date) {
      var year = date.getFullYear(),
          month = date.getMonth(),
          day = date.getDate(),
          pattern = /([+\-]?[0-9]+)\s*(d|D|w|W|m|M|y|Y)?/g,
          matches = pattern.exec(offset);

      while (matches) {
        switch (matches[2] || "d") {
          case "d":
          case "D":
            day += parseInt(matches[1], 10);
            break;
          case "w":
          case "W":
            day += parseInt(matches[1], 10) * 7;
            break;
          case "m":
          case "M":
            month += parseInt(matches[1], 10);
            day = Math.min(day, getDaysInMonth(year, month));
            break;
          case "y":
          case "Y":
            year += parseInt(matches[1], 10);
            day = Math.min(day, getDaysInMonth(year, month));
            break;
        }
        matches = pattern.exec(offset);
      }
      return new Date(year, month, day);
    },
    refDate = (refDate == null || refDate === "" ? new Date() : refDate ),
    newDate = (date == null || date === "" ? 
      defaultDate :
      (typeof date === "string" ? 
        offsetString(date, refDate) : 
        (typeof date === "number" ? 
          (isNaN(date) ? 
            defaultDate : 
            offsetNumeric(date)
          ) : 
          new Date(date.getTime())
        )
      )
    );

    newDate = (newDate && newDate.toString() === "Invalid Date" ? defaultDate : newDate);
    if (newDate) {
      newDate.setHours(0);
      newDate.setMinutes(0);
      newDate.setSeconds(0);
      newDate.setMilliseconds(0);
    }
    return newDate;
}