JS计算当前日期的周次

函数：

function getweek(z){//参数z是"2018-05-07 15:12:36"或者"2018/05/07 15:12:36"
	var day11=Date.parse(z);
	var day11=new Date(day11);
	day11.setMonth(0);
	day11.setDate(1);
	day11.setHours(0);
	day11.setMinutes(0);
	day11.setSeconds(0);//到这里就得到该年的一月一日

	var day11mill=day11.getTime();
	var ori_day=day11.getDay();//该年的一月一日是星期几
	var fill1=0;//与星期日相隔的毫秒数
	if(ori_day!==0){
	    fill1=ori_day*60*60*24*1000;
	}
		
	var now=Date.parse(z);
	now=new Date(now);
	now.setHours(0);
	now.setMinutes(0);
	now.setSeconds(0);
	var nowmill=now.getTime();
	var now_day=now.getDay();
	var fill2=0;
	if(now_day!==0){
	    fill2=(7-now_day)*60*60*24*1000;
	}

	var cha2=(nowmill-day11mill+fill1+fill2)/(60*60*24*1000);
	var week=Math.ceil(cha2/7);
	if(week<10){
	    week="0"+week;
	}
	var year=now.getFullYear().toString();
	year=year.substring(2); 
	return year+week;			
}

例如：var x=getweek("2018-05-07 15:12:36");//结果是1819