js如何获取本日 本周 本月的时间代码

function showToDay()     
{     
    var Nowdate=new Date();     
    M=Number(Nowdate.getMonth())+1     
    alert(Nowdate.getMonth()+"月,"+Nowdate.getDate()+"号,星期"+Nowdate.getDay());     
    return Nowdate.getYear()+"-"+M+"-"+Nowdate.getDate();     
}

function showTomorrow()
{     
    var tom=new Date();     
    tom.setDate(tom.getDate()+1);     
    M=Number(tom.getMonth())+1     
    return tom.getYear()+"-"+M+"-"+tom.getDate();     
}

function showWeekFirstDay()     
{     
    var Nowdate=new Date();     
    var WeekFirstDay=new Date(Nowdate-(Nowdate.getDay()-1)*86400000);     
    M=Number(WeekFirstDay.getMonth())+1     
    return WeekFirstDay.getYear()+"-"+M+"-"+WeekFirstDay.getDate();     
}

function showWeekLastDay()     
{     
    var Nowdate=new Date();     
    var WeekFirstDay=new Date(Nowdate-(Nowdate.getDay()-1)*86400000);     
    var WeekLastDay=new Date((WeekFirstDay/1000+6*86400)*1000);     
    M=Number(WeekLastDay.getMonth())+1     
    return WeekLastDay.getYear()+"-"+M+"-"+WeekLastDay.getDate();     
}

function showMonthFirstDay()     
{     
    var Nowdate=new Date();     
    var MonthFirstDay=new Date(Nowdate.getYear(),Nowdate.getMonth(),1);     
    M=Number(MonthFirstDay.getMonth())+1     
    return MonthFirstDay.getYear()+"-"+M+"-"+MonthFirstDay.getDate();     
}

function showMonthLastDay()     
{     
    var Nowdate=new Date();     
    var MonthNextFirstDay=new Date(Nowdate.getYear(),Nowdate.getMonth()+1,1);     
    var MonthLastDay=new Date(MonthNextFirstDay-86400000);     
    M=Number(MonthLastDay.getMonth())+1     
    return MonthLastDay.getYear()+"-"+M+"-"+MonthLastDay.getDate();     
}

function getTime(s)
{     
    var time1=document.getElementById("num1");     
    var time2=document.getElementById("num2");     
    if(s==1){     
        //本日时间     
        time1.value=showToDay();     
        //alert("今日:"+time1.value);     
        time2.value=showTomorrow();     
        //alert("今日:"+time2.value);     
    }else if(s==2){     
        //本周时间     
        time1.value=showWeekFirstDay();     
        //alert("本月第一天:"+time1.value);     
        time2.value=showWeekLastDay();     
        //alert("本月最后一天:"+time2.value);     
    }else{     
        //本月时间     
        time1.value=showMonthFirstDay();     
        //alert("本月第一天"+time1.value);     
        time2.value=showMonthLastDay();     
        //alert("本月最后一天"+time2.value);         }
}