function onTimezoneOffsetChange(changeHandler){

  function getTimezoneOffset(){
    const d = new Date()
    return d.getTimezoneOffset()
  }

  function getNextHourOffset(){
    const d = new Date()
    return (1000*60*60) - (((d.getMinutes()*60) + d.getSeconds()) * 1000) + d.getMilliseconds()
  }

  let lastTZOffset = getTimezoneOffset()
  let timeoutRef

  function scheduleCheck(){
    timeoutRef = setTimeout(function(){
      const tzOffset = getTimezoneOffset()
      if(lastTZOffset !== tzOffset){
        lastTZOffset = tzOffset
        changeHandler(tzOffset)
      }
      scheduleCheck()
    }, getNextHourOffset()+1) // run 1ms after end of hour
  }

  scheduleCheck()
}

onTimezoneOffsetChange(function(tzOffset){
  console.log("timezone changed")
  // location.reload() or refresh somehow
})