Get all sundays a month or get "specific day of week" in a month using dayjs (In the future)

sundaysInMonth.js

func getSundays {
  let startTime = 1234294232 // unix timestamp of a day in paricular
  const startTimeObj =  dayjs.unix(startTime)
  const startOfDay = dayjs().startOf('day',startTimeObj)
  const endOfMonth = dayjs().endOf('month',startTimeObj)
  const days = []
  let pastEndMonth = false
  let newDay = startOfDay.add(7, 'days')

  do {
    let diff = endOfMonth.diff(newDay,'days')
    if (diff < 1) {
      pastEndMonth = true
    }else{
      pastEndMonth = false
      days.push(newDay)
      newDay = newDay.add(7, 'days')
    }
  } while (pastEndMonth === false);
  console.log(days)
}