initDateTimeWithTimezone

gistfile1.js

/**
 * Return a date object with the specified date and timezone respecting DST
 * @param {string} dateStr - Date string in 'YYYY-MM-DD' or 'YYYY-MM-DD[T| ]HH:MM:SS' format
 * @param {string} timezone - IANA timezone string (e.g., 'Europe/Paris')
 * @returns {Date} - Date object adjusted for the specified timezone
 */
function initDateTimeWithTimezone(dateStr, timezone) {
  let year, month, day, hours = '00', minutes = '00', seconds = '00';
  dateStr = dateStr.trim();
  
  // Check if time is included in the date string
  if (dateStr.includes('T') || dateStr.includes(' ')) {
    // Parse date with time (YYYY-MM-DDT00:00:00 or YYYY-MM-DD 00:00:00)
    const parts = dateStr.replace('T', ' ').split(' ');
    
    // Parse date components
    [year, month, day] = parts[0].split('-');
    
    // Parse time components if provided
    if (parts[1]) {
      const timeParts = parts[1].split(':');
      hours = timeParts[0] || '00';
      minutes = timeParts[1] || '00';
      seconds = timeParts[2] || '00';
    }
  } else {
    // Parse date only (YYYY-MM-DD)
    [year, month, day] = dateStr.split('-');
  }
  
  // Create ISO string like '2025-03-30T00:00:00' in target timezone format
  const targetDateISO = `${year}-${month}-${day}T${hours}:${minutes}:${seconds}`;
  
  // Use the date with timezone string format
  const targetTZDate = new Date(`${targetDateISO}Z`);
  
  // Create a formatter that will give us the UTC offset for this specific date and timezone
  const options = {
    timeZone: timezone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
    hour12: false,
    timeZoneName: 'short'
  };
  
  const formatter = new Intl.DateTimeFormat('en-US', options);
  const dateInTZ = formatter.format(targetTZDate);
  
  // Extract timezone offset from the formatted string (like "03/30/2025, 00:00:00 GMT+2")
  const tzOffsetMatch = dateInTZ.match(/GMT([+-])(\d+)/);
  console.log('offset', tzOffsetMatch[0]);
  if (tzOffsetMatch) {
    const sign = tzOffsetMatch[1] === '+' ? -1 : 1; // If GMT+2, we need to subtract 2 hours from UTC
    const hours = parseInt(tzOffsetMatch[2], 10);
    
    // Calculate the UTC time by adjusting for the timezone offset
    // For midnight in Paris (GMT+2), the UTC time would be 10 PM (22:00) the previous day
    const offsetMillis = sign * hours * 60 * 60 * 1000;
    return new Date(targetTZDate.getTime() + offsetMillis);
  }
  
  // Fallback if we couldn't extract the timezone offset
  return targetTZDate;
}