Convert ISO8601 UTC string to UNIX timestamp and vice versa in Javascript

ts2iso.js

const epoch = 1970
const dayInSeconds = 86400
const hourInSeconds = 3600
const minuteInSeconds = 60
const dayInHours = 24

const toInt = v => parseInt(v)
const isLeap = v => ((v % 4 == 0) && (v % 100 != 0)) || (v % 400 == 0)
const pad = (v, n = 2) => v.toString().padStart(n, '0')

function ts2iso(ts) {
  const seconds = ts % minuteInSeconds
  const minutes = parseInt(ts / minuteInSeconds) % minuteInSeconds
  const hours = parseInt(ts / hourInSeconds) % dayInHours
  const daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

  let days = parseInt(ts / dayInSeconds) + 1
  let years = epoch
  let months = 1

  while (days > 365) {
    if (isLeap(years)) {
      if (days === 366) break
      days -= 366
    } else {
      days -= 365
    }
    years++
  }

  if (isLeap(years)) daysInMonths[1] = 29
  while (days > daysInMonths[months - 1]) {
    days -= daysInMonths[months - 1]
    months++
  }

  const date = `${pad(years, 4)}-${pad(months)}-${pad(days)}`
  const time = `${pad(hours)}:${pad(minutes)}:${pad(seconds)}`
  return date + 'T' + time + 'Z'
}

function iso2ts(iso) {
  const [date, time] = iso.split('T')
  const [year, month, day] = date.split('-').map(toInt)
  const [hour, minute, second] = time.replace('Z', '').split(':').map(toInt)
  const daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

  let ts = 0

  for (let i = epoch; i < year; i++) {
    ts += (isLeap(i) ? 366 : 365) * dayInSeconds
  }

  if (isLeap(year)) daysInMonths[1] = 29
  for (let i = 0; i < month - 1; i++) {
    ts += daysInMonths[i] * dayInSeconds
  }

  ts += (day - 1) * dayInSeconds
  ts += hour * hourInSeconds
  ts += minute * minuteInSeconds
  ts += second
  return ts
}

const tests = {
  "10": "1970-01-01T00:00:10Z",
  "63072000": "1972-01-01T00:00:00Z",
  "315532800": "1980-01-01T00:00:00Z",
  "647135999": "1990-07-04T23:59:59Z",
  "951825600": "2000-02-29T12:00:00Z",
  "1273903628": "2010-05-15T06:07:08Z",
  "1609459199": "2020-12-31T23:59:59Z",
  "1709164800": "2024-02-29T00:00:00Z",
  "1909074600": "2030-06-30T18:30:00Z",
  "2214172862": "2040-03-01T00:01:02Z"
}

Object.entries(tests).sort((a, b) => a[0] - b[0]).forEach(([ts, iso]) => {
  ts = parseInt(ts)
  const iso2 = ts2iso(ts)
  const ts2 = iso2ts(iso)
  if (iso2 !== iso) console.log('ts2iso mismatch ', ts, iso, iso2)
  if (ts2 !== ts) console.log('iso2ts mismatch ', iso, ts, ts2)
})
console.log('done')