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"'