Gets the next day that you will send an automatic message to your clients considering timezone, service start time, working days and holidays.

dateToSendNextMessage.js

function nowUTC(offset) {
    let now = new Date;
    let utc_timestamp = Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate(),
        now.getUTCHours(), now.getUTCMinutes(), now.getUTCSeconds(), now.getUTCMilliseconds());

    return new Date(utc_timestamp + offset * 3600 * 1000);
}

function formatDay(day) {
    return day < 10 ? `0${day}` : day;;
}

function formatMonth(month) {
    month = parseInt(month) + 1;
    return month < 10 ? `0${month}` : month;
}

function isYearLeap(year) {
    return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}

function numberOfDaysInMonth(month, isYearLeap) {
    let numberOfDaysInMonth = [];

    if (isYearLeap) {
        numberOfDaysInMonth = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    } else {
        numberOfDaysInMonth = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    }

    return numberOfDaysInMonth[month];
}

function nextWeekDay(weekday) {
    let firstWeekDay = 0;
    let lastWeekDay = 6;

    // Se é o ultimo dia da semana: 0..6
    if (weekday == lastWeekDay) {
        return firstWeekDay;
    }
    return weekday + 1;
}

function nextDay(day, numberOfDaysInMonth) {
    let firstDayOfMonth = 1;

    // Se é o ultimo dia do mês
    if (day == numberOfDaysInMonth) {
        return firstDayOfMonth;
    }
    return day + 1;
}

function nextMonth(day, month, numberOfDaysInMonth) {
    let firstMonthInYear = 0;
    let lastMonthInYear = 11;

    // Se é ultimo dia do mês
    if (day == numberOfDaysInMonth) {
        //Se é o ultimo mes do ano
        if (month == lastMonthInYear) {
            return firstMonthInYear;
        }
        return month + 1;
    }
    return month;
}

function nextYear(day, month, year, numberOfDaysInMonth) {
    let lastMonthInYear = 11;

    // Se é o último dia do mês & ultimo mês do ano: 31/12
    if (day == numberOfDaysInMonth && month == lastMonthInYear) {
        return year + 1;
    }
    return year;
}

function isHoliday(day, month, holidays) {
    day = formatDay(day);
    month = formatMonth(month);

    if (holidays.includes(`${day}/${month}`)) {
        return true;
    }
    return false;
}

function isWorkDays(weekday, workdays) {
    if (workdays.includes(weekday)) {
        return true;
    }
    return false;
}

function dateToSendNextMessage(timezone, serviceStartTime, workingDays, holidays) {
    let today = nowUTC(timezone);
    let day = today.getUTCDate();
    let weekday = today.getUTCDay();
    let month = today.getUTCMonth();
    let year = today.getUTCFullYear();
    let _numberOfDaysInMonth = numberOfDaysInMonth(month, isYearLeap(year));

    while (true) {
        year = nextYear(day, month, year, _numberOfDaysInMonth);
        month = nextMonth(day, month, _numberOfDaysInMonth);
        weekday = nextWeekDay(weekday);
        day = nextDay(day, _numberOfDaysInMonth);
        _numberOfDaysInMonth = numberOfDaysInMonth(month, isYearLeap(year));

        if (!isHoliday(day, month, holidays) && isWorkDays(weekday, workingDays)) {
            day = formatDay(day);
            month = formatMonth(month);

            return `${year}-${month}-${day}T${serviceStartTime}:00Z`; //Format: 2020-09-10T12:25:00.000Z
        }
    }
}

Params format:

• timezone: "-3"
• serviceStartTime: "7:00"
• workingDays: "0,1,2,3,4,5,6" where 0 is Sunday
• holidays: "01/01|21/04|01/05|07/09|12/10|02/11|15/11|25/12" in which these are brazilian national holidays