Getting a time difference across timezones with moment-timezone

Question

I am given a string representing a time, and a timezone ID. I need to determine whether the time in question occurs within the next half hour, but the computer I'm running on is in a different timezone than the one where the string was captured. Minor note: it's OK (still happensSoon === true) if the event happened in the past). Just trying to distinguish things which happen more than a half hour in the future from the rest.

This seems like it ought to be straightforward, but I'm getting nowhere with it.

const moment = require('moment-timezone')

const hm = s => moment(s).format('HH:mm')

const happensSoon = (then, timezoneId) => {
  console.log(`then:`, then)             // 2018-10-04T16:39:52-07:00
  console.log(`timezoneId:`, timezoneId) // America/New_York
  const localNow = moment()
    .clone()
    .tz(timezoneId)
  const localThen = moment(then)
    .clone()
    .tz(timezoneId)
  const diff = localThen.diff(localNow, 'minutes')
  console.log(`localThen:`, hm(localThen)) // 19:39
  console.log(`localNow:`, hm(localNow))   // 16:24
  console.log(`then:`, hm(then))           // 16:39
  console.log(`diff:`, diff)               // 194
  return diff <= 30
}

Running in "America/Los_Angeles" timezone. My "local" is intended to represent the New York time. So, 16:39 is the input value for then and I expect the comparison to be around that time (Oh, I'm running this at about 13:20 developer-local time). So, basically, in the code above, I want to be comparing 16:39 with 16:20, big-apples-to-big-apples. I don't want to hack my way into this; I want a solution I understand. Thanks!

Answer 1

This does the job for me:

const happensSoon = (then, timezoneId) => {
  const thenThere = moment.tz(then, timezoneId)
  const nowThere = moment().tz(timezoneId)
  const diff = thenThere.diff(nowThere, 'minutes')
  return diff <= 30
}

Given a time string then without timezone information and a timezoneId, it creates a moment at that time and timezone, then creates a new moment and translates it to the same timezone, then diffs them.