getTimeZoneOffset callback passes inconsistent value type

[mediafreakch]

Expected behaviour

time.getTimezoneOffset should take a parameter timestamp that is milliseconds since January 1 1970 according to docs

Actual behaviour

Effectively timestamp is sometimes the number of milliseconds, but also a Date() object. I think it depends on what kind of "thing" is being transformed (label, axis, value, ...).

Live demo with steps to reproduce

https://jsfiddle.net/tw6gujfv/ (observe in console)

Product version

Highcharts JS v8.0.0 (2019-12-10)

Affected browser(s)

all

[mediafreakch]

On this occasion, maybe also the return type of the function should be documented correctly: It should be number and not string.

[pawelfus]

Thanks for reporting @mediafreakch

FYI @TorsteinHonsi, @bre1470

[bre1470]

Looks like the documentation is incorrect and we will investigate, why it sometimes provide timestamp instead of Date instance. As timezone information is only available on the Date instance and timestamp is always GMT, it makes more sense to have always a Date instance for this callback.

[mediafreakch]

@bre1470 We use this callback to get around the limitation that timezones are only supported with Moment.js. We use date-fns-tz (those Date instances have no timezone information, but it works regardless).

We use the following callback to transform to a highcharts compatible format:

export function getTimezoneOffset(timestamp: number | Date): number {
  let serverUTCTime: number

  if (typeof timestamp === 'object') {
    serverUTCTime = timestamp.getTime()
  } else {
    serverUTCTime = timestamp
  }

  const stationUnixTime = utcToZonedTime(serverUTCTime, this.timeZone).getTime()
  const utcUnixTime = utcToZonedTime(serverUTCTime, 'UTC').getTime()

  // for highcharts, we need the reverse-offset: how  much does UTC offset from stationTime?
  // For Date.UTC(2020, 0, 1) Europe/Zurich it is -01:00
  const offsetInMinutes = Math.floor((utcUnixTime - stationUnixTime) / 1000 / 60)
  return offsetInMinutes
}

An example: https://codesandbox.io/s/morning-thunder-9yw3n

[bre1470]

@mediafreakch
Thank you for the additional example of implementation! So in your use case a number type would be clearly more beneficial.