date-utils.ts

date-utils.ts

const datePartMapping = [
  ["year", "UTCFullYear"],
  ["month", "UTCMonth"],
  ["day", "UTCDate"],
  ["hour", "UTCHours"],
  ["minute", "UTCMinutes"],
  ["second", "UTCSeconds"],
] as const;

const dateValuesKeys = datePartMapping.map((m) => m[0]);
type DateValuesKeys = typeof dateValuesKeys extends Array<infer K> ? K : never;

const defaultDateValues = Object.fromEntries(
  datePartMapping.map((m) => [m[0], 0])
) as Record<DateValuesKeys, number>;
type DateValues = typeof defaultDateValues;

function getInt(n: string) {
  return parseInt(n, 10);
}

function getUTCDate(values: DateValues) {
  const date = new Date(
    Date.UTC(
      values.year,
      // Date.UTC accepts 0 as the first month.
      values.month - 1,
      values.day,
      values.hour,
      values.minute,
      values.second
    )
  );

  // Years 0-99 are Date.UTC shorthand for 19xx. date.setUTCFullYear does not do this.
  date.setUTCFullYear(values.year);

  return date;
}

let cachedTimeZone: string;
let cachedFormatter: Intl.DateTimeFormat;

/**
 * Gets date parts at the given time zone.
 *
 * @param date
 * @param timeZone IANA time zone ID.
 * @return \{ year: 2024, month: 12, ...etc. }.
 */
export function getDateValuesAtZone(date: Date, timeZone: string) {
  const result = { ...defaultDateValues };

  // en-GB is good enough for getting numeric values for date parts.
  const formatter =
    timeZone === cachedTimeZone
      ? cachedFormatter
      : new Intl.DateTimeFormat("en-GB", {
          day: "2-digit",
          era: "short",
          hour: "2-digit",
          hour12: false,
          minute: "2-digit",
          month: "2-digit",
          second: "2-digit",
          timeZone,
          year: "numeric",
        });
  let era = "";
  cachedTimeZone = timeZone;
  cachedFormatter = formatter;

  for (const part of formatter.formatToParts(date)) {
    if (part.type in result) {
      (result as any)[part.type] = parseInt(part.value);
    } else if (part.type === "era") {
      era = part.value;
    }
  }

  result.year = era === "BC" ? -(result.year - 1) : result.year;

  return result;
}

function getDiffDateValues(target: DateValues, current: DateValues) {
  const result = { ...defaultDateValues, isDiff: false };

  for (const key of dateValuesKeys) {
    const diff = current[key] - target[key];
    if (diff !== 0) {
      result.isDiff = true;
    }
    result[key] = diff;
  }

  return result;
}

function adjustDate(
  date: Date,
  diffValues: ReturnType<typeof getDiffDateValues>
) {
  for (const [type, utcMethodName] of datePartMapping) {
    const setMethod = `set${utcMethodName}` as const;
    const getMethod = `get${utcMethodName}` as const;
    date[setMethod](date[getMethod]() - diffValues[type]);
  }

  return date;
}

/**
 * Gets a Date set to the observed date & time at the location of the time zone.
 *
 * @param timeZone IANA time zone ID.
 * @param date 'YYYY-MM-DD'
 * @param time 'hh:mm:ss'
 */
export function getDateAtZone(timeZone: string, date: string, time = "0") {
  const isNegativeYear = date.startsWith("-");
  const [inputYear, inputMonth, inputDay] = (
    isNegativeYear ? date.slice(1) : date
  )
    .split("-")
    .map(getInt);
  const [inputHour, inputMinute = 0, inputSecond = 0] = time
    .split(":")
    .map(getInt);
  const inputValues = {
    year: isNegativeYear ? -inputYear : inputYear,
    month: inputMonth,
    day: inputDay,
    hour: inputHour,
    minute: inputMinute,
    second: inputSecond,
  };
  let resultDate = getUTCDate(inputValues);
  const diffFromUTCOffset = getDiffDateValues(
    inputValues,
    getDateValuesAtZone(resultDate, timeZone)
  );

  if (!diffFromUTCOffset.isDiff) {
    return resultDate;
  }

  resultDate = adjustDate(resultDate, diffFromUTCOffset);

  const diffFromTimeChange = getDiffDateValues(
    inputValues,
    getDateValuesAtZone(resultDate, timeZone)
  );

  if (!diffFromTimeChange.isDiff) {
    return resultDate;
  }

  return adjustDate(resultDate, diffFromTimeChange);
}