看一下源码：
Date.prototype.toJSON = function() {
return '"' this.getUTCFullYear() '-'
(this.getUTCMonth() 1).toPaddedString(2) '-'
this.getUTCDate().toPaddedString(2) 'T'
this.getUTCHours().toPaddedString(2) ':'
this.getUTCMinutes().toPaddedString(2) ':'
this.getUTCSeconds().toPaddedString(2) 'Z"';
}; 其实就是返回Date的JSON字符串，下面给出示例：
new Date(1969, 11, 31, 19).toJSON(); //-> '"1969-12-31T19:00:00"'