Function DateToDay

Conversions/DateToDay.js:28–70

(date)

]

const DateToDay = (date) => {
  // firstly, check that input is a string or not.
  const dateStruct = parseDate(date)
  let year = dateStruct.year
  let month = dateStruct.month
  let day = dateStruct.day

  // In case of Jan and Feb:
  // Year: we consider it as previous year
  // e.g., 1/1/1987 here year is 1986 (-1)
  // Month: we consider value as 13 & 14 respectively
  if (month < 3) {
    year--
    month += 12
  }

  // divide year into century and the last two digits of the century
  const yearDigits = year % 100
  const century = Math.floor(year / 100)

  /*
  In mathematics, remainders of divisions are usually defined to always be positive;
  As an example, -2 mod 7 = 5.
  Many programming languages including JavaScript implement the remainder of `n % m` as `sign(n) * (abs(n) % m)`.
  This means the result has the same sign as the numerator. Here, `-2 % 7 = -1 * (2 % 7) = -2`.

  To ensure a positive numerator, the formula is adapted: `- 2 * century` is replaced with `+ 5 * century`
  which does not alter the resulting numbers mod 7 since `7 - 2 = 5`

  The following example shows the issue with modulo division:
  Without the adaption, the formula yields `weekDay = -6` for the date 2/3/2014;
  With the adaption, it yields the positive result `weekDay = 7 - 6 = 1` (Sunday), which is what we need to index the array
  */
  const weekDay =
    (day +
      Math.floor((month + 1) * 2.6) +
      yearDigits +
      Math.floor(yearDigits / 4) +
      Math.floor(century / 4) +
      5 * century) %
    7
  return daysNameArr[weekDay] // name of the weekday
}

// Example : DateToDay("18/12/2020") => Friday