一個被我忽視的Javascript問題

前幾日使用到Javascript的Date初始化一個日期
var dt = new Date(2005,8,31)
結果發現dt.getDate()為1

仔細查了一下Javascript參考才發現，原來如此

Versions prior to JavaScript 1.3:

new Date()
new Date(milliseconds)
new Date(dateString)
new Date(yr_num, mo_num, day_num[, hr_num, min_num, sec_num])

Parameters

milliseconds	Integer value representing the number of milliseconds since 1 January 1970 00:00:00.
dateString	String value representing a date. The string should be in a format recognized by the Date.parse method.
yr_num, mo_num, day_num	Integer values representing part of a date. As an integer value, the month is represented by 0 to 11 with 0=January and 11=December.
hr_num, min_num, sec_num, ms_num	Integer values representing part of a date.