SkyMoney/web/src/utils/timezone.ts

/**
 * Timezone utility functions for consistent date handling across the application.
 * 
 * All dates should be:
 * 1. Stored in the backend as UTC ISO strings
 * 2. Displayed to users in their saved timezone
 * 3. Input from users interpreted in their saved timezone
 * 
 * The user's timezone is stored in the database and should be fetched from the dashboard.
 */

/**
 * Get today's date in the user's timezone as YYYY-MM-DD format.
 * This should be used for date inputs to ensure consistency with user's timezone.
 * 
 * @param userTimezone - IANA timezone string (e.g., "America/New_York")
 * @returns Date string in YYYY-MM-DD format
 */
export function getTodayInTimezone(userTimezone: string): string {
  const now = new Date();
  // Use Intl.DateTimeFormat to get the date in the user's timezone
  const formatter = new Intl.DateTimeFormat('en-CA', { // en-CA gives YYYY-MM-DD format
    timeZone: userTimezone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
  });
  return formatter.format(now);
}

/**
 * Convert a date input string (YYYY-MM-DD) to an ISO string that represents
 * midnight in the user's timezone.
 * 
 * This should be used when sending date-only data to the backend.
 * 
 * @param dateString - Date in YYYY-MM-DD format
 * @param userTimezone - IANA timezone string
 * @returns ISO string representing midnight in the user's timezone
 */
export function dateStringToUTCMidnight(dateString: string, userTimezone: string): string {
  // Parse the date string as-is (YYYY-MM-DD)
  const [year, month, day] = dateString.split('-').map(Number);
  
  // Create a date object representing midnight in the user's timezone
  // We format a string that includes timezone info
  const dateStr = `${year}-${String(month).padStart(2, '0')}-${String(day).padStart(2, '0')}T00:00:00`;
  
  // Get the date/time string in the user's timezone to calculate offset
  const tzDate = new Date(new Date(dateStr).toLocaleString('en-US', { timeZone: userTimezone }));
  const utcDate = new Date(new Date(dateStr).toLocaleString('en-US', { timeZone: 'UTC' }));
  const offset = tzDate.getTime() - utcDate.getTime();
  
  // Create final date adjusted for timezone
  const adjustedDate = new Date(Date.UTC(year, month - 1, day, 0, 0, 0) - offset);
  
  return adjustedDate.toISOString();
}

/**
 * Format an ISO date string for display in the user's timezone.
 * 
 * @param isoString - ISO date string from backend
 * @param userTimezone - IANA timezone string
 * @param options - Intl.DateTimeFormatOptions for formatting
 * @returns Formatted date string
 */
export function formatDateInTimezone(
  isoString: string,
  userTimezone: string,
  options: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric' }
): string {
  const date = new Date(isoString);
  return new Intl.DateTimeFormat('en-US', {
    ...options,
    timeZone: userTimezone,
  }).format(date);
}

/**
 * Convert an ISO string to YYYY-MM-DD format in the user's timezone.
 * This is useful for populating date inputs.
 * 
 * @param isoString - ISO date string from backend
 * @param userTimezone - IANA timezone string
 * @returns Date string in YYYY-MM-DD format
 */
export function isoToDateString(isoString: string, userTimezone: string): string {
  const date = new Date(isoString);
  const formatter = new Intl.DateTimeFormat('en-CA', {
    timeZone: userTimezone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
  });
  return formatter.format(date);
}

/**
 * Get the current date and time as an ISO string in UTC.
 * This should be used for timestamps (not date-only fields).
 * 
 * @returns ISO string in UTC
 */
export function getCurrentTimestamp(): string {
  return new Date().toISOString();
}

/**
 * Compare two date strings (YYYY-MM-DD) to determine if date1 is before date2.
 * This is timezone-safe because it compares date strings directly.
 * 
 * @param date1 - First date string
 * @param date2 - Second date string
 * @returns true if date1 is before date2
 */
export function isDateBefore(date1: string, date2: string): boolean {
  return date1 < date2;
}

/**
 * Compare two date strings (YYYY-MM-DD) to determine if date1 is after date2.
 * This is timezone-safe because it compares date strings directly.
 * 
 * @param date1 - First date string
 * @param date2 - Second date string
 * @returns true if date1 is after date2
 */
export function isDateAfter(date1: string, date2: string): boolean {
  return date1 > date2;
}

/**
 * Add days to a date string, accounting for the user's timezone.
 * 
 * @param dateString - Date in YYYY-MM-DD format
 * @param days - Number of days to add
 * @param userTimezone - IANA timezone string
 * @returns New date string in YYYY-MM-DD format
 */
export function addDaysToDate(dateString: string, days: number, userTimezone: string): string {
  const baseISO = dateStringToUTCMidnight(dateString, userTimezone);
  const base = new Date(baseISO);
  base.setUTCDate(base.getUTCDate() + days);
  return isoToDateString(base.toISOString(), userTimezone);
}

/**
 * Get the user's browser timezone as a fallback.
 * This should only be used when the backend timezone is not available.
 * 
 * @returns IANA timezone string
 */
export function getBrowserTimezone(): string {
  return Intl.DateTimeFormat().resolvedOptions().timeZone;
}