今日、今週、今月のタイムコードをjsで取得する方法

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();     
}