将时间转换为时间戳

var date = new Date('2020-05-05');
// 有三种方式获取
var time1 = date.getTime();
var time2 = date.valueOf();
var time3 = Date.parse(date);
console.log(time1);//1588636800000
console.log(time2);//1588636800000
console.log(time3);//1588636800000