index.ts

/**
 * Returns the day of the year.
 * @param date - Date object.
 * @param startAtZero - Whether the day of the year should be zero-based.
 * @returns Day of year,
 */
export function getDayOfYear(date: Date, startAtZero = false): number {
  const year = date.getFullYear()

  const januaryFirst = new Date(year, 0, 1)
  const givenDate = new Date(year, date.getMonth(), date.getDate())

  return (
    Math.round((givenDate.getTime() - januaryFirst.getTime()) / 86_400_000) +
    (startAtZero ? 0 : 1)
  )
}

/**
 * Returns a date string in the modern Julian format. (i.e. YYYYDDD)
 * ex. 2023-01-01 => '2023001'
 * @param date - Date object.
 * @returns Date string in modern Julian format
 */
export function toModernJulianDate(date: Date): string {
  const dayOfYear = getDayOfYear(date)
  return (date.getFullYear() * 1000 + dayOfYear).toString()
}

/**
 * Returns a date string in the modern Julian format with a two-digit year. (i.e. YYDDD)
 * ex. 2023-01-01 => '23001'
 * @param date - Date object.
 * @returns Date string in modern Julian format with a two-digit year
 */
export function toShortModernJulianDate(date: Date): string {
  return toModernJulianDate(date).slice(2)
}

/**
 * Converts a modern Julian date into a Javascript Date object.
 * ex. '2023001' => 2023-01-01
 * @param modernJulianDate - A five-digit (short) or seven-digit modern Julian date. Note that five-digit dates will be assumed to be in the current century.
 * @returns Date object.
 */
export function fromModernJulianDate(modernJulianDate: string): Date {
  let sevenDigitModernJulianDate = modernJulianDate

  if (modernJulianDate.length === 5) {
    sevenDigitModernJulianDate =
      Math.floor(new Date().getFullYear() / 100).toString() + modernJulianDate
  }

  if (sevenDigitModernJulianDate.length !== 7) {
    throw new Error(`Invalid modern Julian date: ${modernJulianDate}`)
  }

  const year = Number.parseInt(sevenDigitModernJulianDate.slice(0, 4))
  const days = Number.parseInt(sevenDigitModernJulianDate.slice(4))

  if (Number.isNaN(year) || Number.isNaN(days) || days > 366) {
    throw new TypeError(`Invalid modern Julian date: ${modernJulianDate}`)
  }

  // eslint-disable-next-line sonarjs/no-identical-expressions
  const date = new Date(year, 1 - 1, 1)

  date.setDate(days)

  return date
}

export default toModernJulianDate