Date calculator

// These pure functions are the same arithmetic used by this browser tool.
// No API operation adds/subtracts dates; call shiftCalendarDate locally.
const DAY_MS = 86400000;
function businessDateDay(value) {
    if (!/^\d{4}-\d{2}-\d{2}$/.test(value))
        return null;
    const [year, month, day] = value.split('-').map(Number);
    if (year < 1)
        return null;
    const date = new Date(0);
    date.setUTCFullYear(year, month - 1, day);
    if (date.getUTCFullYear() !== year || date.getUTCMonth() !== month - 1 || date.getUTCDate() !== day)
        return null;
    return date.getTime() / DAY_MS;
}
function businessDateAfter(value, days) {
    const start = businessDateDay(value);
    if (start === null || !Number.isInteger(days))
        throw new Error('Enter a valid date.');
    return new Date((start + days) * DAY_MS).toISOString().slice(0, 10);
}
const DAY = 86_400_000;
function calendarDay(value) {
    const day = businessDateDay(value);
    if (day === null || Number(value.slice(0, 4)) < 1000)
        throw new Error('Enter a valid date between 1000-01-01 and 9999-12-31.');
    return day;
}
function isoDay(day) {
    if (!Number.isSafeInteger(day) || day < calendarDay('1000-01-01') || day > calendarDay('9999-12-31')) {
        throw new Error('The result must be between 1000-01-01 and 9999-12-31.');
    }
    return new Date(day * DAY).toISOString().slice(0, 10);
}
function countCalendarDays(start, end, includeStart = false, includeEnd = true) {
    const difference = calendarDay(end) - calendarDay(start);
    const count = Math.max(0, Math.abs(difference) + Number(includeStart) + Number(includeEnd) - 1);
    return { start, end, includeStart, includeEnd, days: count ? count * (difference < 0 ? -1 : 1) : 0,
        weeks: Math.floor(count / 7), remainingDays: count % 7 };
}
/** Shift the original date once. Clamp a missing target-month day to its last day. */
function shiftCalendarDate(start, amount, unit, operation = 'add') {
    const day = calendarDay(start);
    if (!Number.isSafeInteger(amount) || amount < 0 || amount > 10_000_000)
        throw new Error('Enter a whole number from 0 to 10,000,000.');
    if (!['days', 'weeks', 'months', 'years'].includes(unit) || !['add', 'subtract'].includes(operation))
        throw new Error('Choose a date operation and unit.');
    const offset = amount * (operation === 'subtract' ? -1 : 1);
    let result;
    let clamped = false;
    if (unit === 'days' || unit === 'weeks')
        result = isoDay(day + offset * (unit === 'weeks' ? 7 : 1));
    else {
        const [year, month, date] = start.split('-').map(Number);
        const targetMonth = year * 12 + month - 1 + offset * (unit === 'years' ? 12 : 1);
        const targetYear = Math.floor(targetMonth / 12);
        const monthIndex = ((targetMonth % 12) + 12) % 12;
        if (targetYear < 1000 || targetYear > 9999)
            throw new Error('The result must be between 1000-01-01 and 9999-12-31.');
        const lastDay = new Date(Date.UTC(targetYear, monthIndex + 1, 0)).getUTCDate();
        const targetDate = Math.min(date, lastDay);
        clamped = targetDate !== date;
        result = `${targetYear}-${String(monthIndex + 1).padStart(2, '0')}-${String(targetDate).padStart(2, '0')}`;
    }
    return { start, amount, unit, operation, date: result, clamped, days: calendarDay(result) - day };
}
function calendarDateDetails(value) {
    const day = calendarDay(value);
    const date = new Date(day * DAY);
    const year = date.getUTCFullYear();
    const weekday = date.toLocaleDateString('en-US', { weekday: 'long', timeZone: 'UTC' });
    const thursday = new Date((day + 4 - (date.getUTCDay() || 7)) * DAY);
    const weekYear = thursday.getUTCFullYear();
    const week = Math.ceil(((thursday.getTime() - Date.UTC(weekYear, 0, 1)) / DAY + 1) / 7);
    return { weekday, week, weekYear, dayOfYear: day - calendarDay(`${year}-01-01`) + 1,
        leapYear: year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0),
        daysInMonth: new Date(Date.UTC(year, date.getUTCMonth() + 1, 0)).getUTCDate() };
}
function calendarCountSummary(result) {
    return [`${result.days} calendar ${Math.abs(result.days) === 1 ? 'day' : 'days'}`,
        `From ${result.start} to ${result.end}.`,
        `Start ${result.includeStart ? 'included' : 'excluded'}. End ${result.includeEnd ? 'included' : 'excluded'}. Weekends and holidays included.`,
        `${result.weeks} complete ${result.weeks === 1 ? 'week' : 'weeks'} and ${result.remainingDays} remaining ${result.remainingDays === 1 ? 'day' : 'days'}${result.days < 0 ? ', backwards' : ''}.`].join('\n');
}
function calendarShiftSummary(result) {
    return [`${result.date} (${calendarDateDetails(result.date).weekday})`,
        `${result.operation === 'add' ? 'Add' : 'Subtract'} ${result.amount} ${result.amount === 1 ? result.unit.slice(0, -1) : result.unit} ${result.operation === 'add' ? 'to' : 'from'} ${result.start}.`,
        ...(result.clamped ? ['The original day does not exist in the target month; the result uses its last day.'] : [])].join('\n');
}