zoned date-fns-utils

zoned-date-fns.js

// import { addDays, subDays } from 'npm:date-fns'; // uncomment for deno
import {addDays,subDays} from "date-fns"

const getCurrentZone = () => Intl?.DateTimeFormat()?.resolvedOptions().timeZone

export function getZonedWeekday(date, zone = getCurrentZone()) {
  const weekdayString = new Date(date).toLocaleString('en-GB', {
    timeZone: zone,
    weekday: 'long',
  })

  const positional = [
    'Monday',
    'Tuesday',
    'Wednesday',
    'Thursday',
    'Friday',
    'Saturday',
    'Sunday',
  ]

  const hasIt = positional.findIndex(
    x => x.toLowerCase() === weekdayString.toLowerCase()
  )

  if (hasIt == -1) {
    return NaN
  }

  return hasIt + 1
}

export function addZonedBusinessDays(date, unit, zone = getCurrentZone()) {
  const isWeekend = dt => {
    const weekday = getZonedWeekday(dt, zone)
    return weekday === 6 || weekday === 7
  }

  let result = addDays(date, unit)
  while (isWeekend(result)) {
    result = addDays(result, unit)
  }
  return result
}

export function subZonedBusinessDays(date, unit, zone = getCurrentZone()) {
  const isWeekend = dt => {
    const weekday = getZonedWeekday(dt, zone)
    return weekday === 6 || weekday === 7
  }

  let result = subDays(date, unit)
  while (isWeekend(result)) {
    result = subDays(result, unit)
  }
  return result
}