取当天0时0秒0分

var time = new Date(new Date().setHours(0, 0, 0, 0));
console.log(time)

取时间戳：

var timeStamp = new Date(new Date().setHours(0, 0, 0, 0)) / 1000;
console.log(timeStamp)

取几天前的时间戳

var timeStamp = new Date(new Date().setHours(0, 0, 0, 0)) / 1000;
//一天是86400秒，n天前的时间戳
var nDayAgo = timeStamp - 86400 * n;
console.log(nDayAgo)