Rsa97

echo (new DateTimeImmutable('first day of January next year'))
    ->diff(new DateTimeImmutable())
    ->format('Осталось дней: %a');
// Осталось дней: 151

const dateByWeekNumber = (year, week) => {
  // Cоздаём дату, гарантированно входящую в первую неделю.
  const date = new Date(year, 0, 7);
  // Откатываемся до первого четверга года
  // По ГОСТ ИСО 8601-2001 первая неделя года должна содержать четверг
  date.setDate(date.getDate() - (date.getDay() + 10) % 7);
  // Переходим в нужную неделю
  date.setDate(date.getDate() + (week - 1) * 7);
  // Откатываемся до понедельника
  date.setDate(date.getDate() - 3);
  return date;
};
dateByWeekNumber(2023, 10);
//  Date Mon Mar 06 2023 00:00:00 GMT+0300 (Москва, стандартное время)
dateByWeekNumber(2020, 1);
// Date Mon Dec 30 2019 00:00:00 GMT+0300 (Москва, стандартное время)

The special area of "Etc" is used for some administrative zones, particularly for "Etc/UTC" which represents Coordinated Universal Time. In order to conform with the POSIX style, those zone names beginning with "Etc/GMT" have their sign reversed from the standard ISO 8601 convention. In the "Etc" area, zones west of GMT have a positive sign and those east have a negative sign in their name (e.g "Etc/GMT-14" is 14 hours ahead of GMT)

const date = new Date();
const asWeekday = new Intl.DateTimeFormat('en-US', {weekday: 'long'});
const today = asWeekday.format(date);
date.setDate(date.getDate() + 1);
const tomorrow = asWeekday.format(date);
console.log(today, tomorrow);
// Friday Saturday

$dateTime = new DateTime('2021-05-18T15:30:00.000-04:00');
var_dump($dateTime);
$dateTime->setTimezone(new DateTimeZone('Europe/Moscow'));
var_dump($dateTime);

// object(DateTime)#1 (3) {
//   ["date"] => string(26) "2021-05-18 15:30:00.000000"
//   ["timezone_type"] => int(1)
//   ["timezone"] => string(6) "-04:00"
// }
//object(DateTime)#1 (3) {
//  ["date"] => string(26) "2021-05-18 22:30:00.000000"
//  ["timezone_type"] => int(3)
//  ["timezone"] => string(13) "Europe/Moscow"
//}

$timestamp = strtotime('2021-05-18T15:30:00.000-04:00');
var_dump($timestamp); // int(1621366200)
date_default_timezone_set('Europe/Moscow');
$dateTime = date('c', $timestamp);
var_dump($dateTime); // string(25) "2021-05-18T22:30:00+03:00"

function getFreeInterval($intervals, $startTime, $endTime)
{
    $result = [];
    $ok = false;
    foreach($intervals as $key => $interval){
        list($start, $end) = explode('-', $interval);
        if ($startTime >= $start && $endTime <= $end) {
            $ok = true;
            if ($startTime > $start) {
                $result[] = "{$start}-{$startTime}";
            }
            if ($endTime < $end) {
                $result[] = "{$endTime}-{$end}";
            }
        } else {
            $result[] = $interval;
        }
    }
    return [
        $ok,
        $result
    ];
}

$intervals = [
  '09:30:00-13:15:00',
  '14:00:00-17:00:00'
];

list($ok, $result) = getFreeInterval($intervals, '09:30:00', '10:15:00');

var_dump($ok, $result);

// bool(true)
// array(2) {
//   [0]=> string(17) "10:15:00-13:15:00"
//   [1]=> string(17) "14:00:00-17:00:00"
// }

(new Date(endDate).getTime() - new Date(startDate).getTime()) / 1000 / 60 / 60

(new Date('2019-11-14T09:07:02.376+00:00') - new Date()) < 3 * 24 * 60 * 60 * 1000

<?php
$begin = new DateTime('09:00');
$end = new DateTime('21:00');
$intervals = array(
  array(new DateTime('10:00'), new DateTime('10:15')),
  array(new DateTime('10:15'), new DateTime('11:45')),
  array(new DateTime('12:30'), new DateTime('16:00'))
);
$time = 90;

function checkInterval($begin, $end, $time) {
  $intervalDiff = $begin->diff($end);
  $intervalMinutes = $intervalDiff->h * 60 + $intervalDiff->i;
  if ($intervalMinutes >= $time) {
    echo $begin->format('H:i'), ' - ', $end->format('H:i'), "\n";
  }
}

$intervalStart = $begin;
foreach ($intervals as $interval) {
  checkInterval($intervalStart, $interval[0], $time);
  $intervalStart = $interval[1];
}
checkInterval($intervalStart, $end, $time);