function getNearDate(date){
    let near_date = new Date(date);
    let dayOfMonth = date.getDate();
    if(dayOfMonth <= 5)
        near_date.setDate(5);
    else if(dayOfMonth <=20)
        near_date.setDate(20);
    else{
        near_date.setDate(5);
        near_date.setMonth(date.getMonth()+1);
    }

    let dayOfWeek = near_date.getDay();
    if(dayOfWeek == 0)
        //Если воскресенье
        near_date.setDate(near_date.getDate()-2);
    else if(dayOfWeek == 6)
        //Если суббота
        near_date.setDate(near_date.getDate()-1);
    return near_date;
}

function createDate(day, month, year){
    //Принимает день и месяц в НОРМАЛЬНОЙ нотации
    let date = new Date;
    date.setFullYear(year);
    date.setMonth(month-1);
    date.setDate(day);
    return date;
}

function getDiffByDays(d1,d2){
    return Math.abs(Math.round((d1 - d2)/(3600*1000*24)));
}

function printDate(date){
    console.log(date.getDate(), date.getMonth()+1, date.getFullYear());
}

function getBeginDateByDatePeriod(datePeriod){
    //2018-06-01/2018-06-30
    let tempStr = datePeriod.slice(0,10);
    let year = tempStr.slice(0,4);
    let month = tempStr.slice(5,7);
    let day = tempStr.slice(8,10);
    return createDate(day,month,year);
}


printDate(getBeginDateByDatePeriod("2018-06-01/2018-06-30"));