date-tools

/**
 * Timezone resolution helpers built on the host ICU database.
 */

import { InvalidTimezoneError } from "../errors/invalid-timezone-error"

/**
 * Validate that the supplied timezone name is recognised by the host ICU database.
 *
 * @param timezone - The IANA timezone name to validate.
 * @throws When the name is not a recognised IANA timezone.
 */
export function validateTimezone(timezone: string): void {
  try {
    new Intl.DateTimeFormat("en-US", { timeZone: timezone })
  } catch {
    throw new InvalidTimezoneError(timezone)
  }
}

/**
 * Resolve the host system's IANA timezone name as reported by the ICU runtime.
 *
 * @returns The host timezone (e.g. `Europe/London`), or `UTC` if the runtime does not report one.
 */
export function resolveHostTimezone(): string {
  const { timeZone } = Intl.DateTimeFormat().resolvedOptions()

  return timeZone === "" ? "UTC" : timeZone
}

/**
 * Format a date in the requested timezone using `en-CA` to produce ISO-style `YYYY-MM-DD HH:mm:ss`.
 *
 * @param date - The instant to format.
 * @param timezone - The IANA timezone in which to render the instant.
 * @returns The formatted local date-time string.
 */
export function formatInTimezone(date: Date, timezone: string): string {
  const formatter = new Intl.DateTimeFormat("en-CA", {
    timeZone: timezone,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    hour12: false,
  })

  return formatter.format(date).replace(", ", " ")
}