TLDR

My docker container vs my development laptop where running in different timezones.

• Prod = UTC
• Dev = CDT

This was the cause of my problem. Below are a few solutions

Functions

export function dateToISOTimezone(
	date: string,
	time: string,
	timeZone: string
) {
	console.log({ date, time, timeZone })
	console.log(new Date(`${date}T${time}`).toLocaleString("en-US"))
	console.log(new Date(`${date}T${time}`).toLocaleString("en-US", { timeZone }))

	const dateObj = new Date(`${date}T${time}`)
	const localized = dateObj.toLocaleString("en-US", { timeZone })
	const localDateObj = new Date(localized)
	const utcISOString = localDateObj.toISOString()

	console.log({ dateObj, localized, localDateObj, utcISOString })
	return utcISOString
}

const formattedDate = new Intl.DateTimeFormat('en-US', {
    timeZone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
  }).format(localDate)

Dev

personal laptop

❯ date
Sat 05 Apr 2025 01:09:50 PM CDT

n:dev: { date: '2025-04-30', time: '10:00:00', timeZone: 'America/Chicago' }
n:dev: 4/30/2025, 10:00:00 AM
n:dev: 4/30/2025, 10:00:00 AM
n:dev: {
n:dev:   dateObj: 2025-04-30T15:00:00.000Z,
n:dev:   localized: '4/30/2025, 10:00:00 AM',
n:dev:   localDateObj: 2025-04-30T15:00:00.000Z,
n:dev:   utcISOString: '2025-04-30T15:00:00.000Z'
n:dev: }

n:dev: { date: '2025-04-29', time: '10:00:00', timeZone: 'America/Detroit' }
n:dev: 4/29/2025, 10:00:00 AM
n:dev: 4/29/2025, 11:00:00 AM
n:dev: {
n:dev:   dateObj: 2025-04-29T15:00:00.000Z,
n:dev:   localized: '4/29/2025, 11:00:00 AM',
n:dev:   localDateObj: 2025-04-29T16:00:00.000Z,
n:dev:   utcISOString: '2025-04-29T16:00:00.000Z'
n:dev: }

Prod

In a docker container

/app $ date
Sat Apr  5 18:09:51 UTC 2025

{ date: '2025-04-23', time: '10:00:00', timeZone: 'America/Chicago' }
4/23/2025, 10:00:00 AM
4/23/2025, 5:00:00 AM
{
  dateObj: 2025-04-23T10:00:00.000Z,
  localized: '4/23/2025, 5:00:00 AM',
  localDateObj: 2025-04-23T05:00:00.000Z,
  utcISOString: '2025-04-23T05:00:00.000Z'
}

why when localized does it shift 5 hours? The command being

const localized = dateObj.toLocaleString("en-US", { timeZone })

Tested with a different timezone

{ date: '2025-04-23', time: '10:00:00', timeZone: 'America/Detroit' }
4/23/2025, 10:00:00 AM
4/23/2025, 6:00:00 AM
{
  dateObj: 2025-04-23T10:00:00.000Z,
  localized: '4/23/2025, 6:00:00 AM',
  localDateObj: 2025-04-23T06:00:00.000Z,
  utcISOString: '2025-04-23T06:00:00.000Z'
}

GPT Answer

YES — 100% that's a key part of the problem.

You're seeing:

• Production container: UTC timezone

• Development machine: CDT (Central Daylight Time, aka America/Chicago)

This means that any usage of new Date('2025-04-30T10:00:00') will be interpreted differently depending on the environment:

🧠 What This Means in Practice

When you run:

new Date('2025-04-30T10:00:00')

It creates a Date object based on the local timezone of the system:

So even though you passed in "2025-04-30T10:00:00", the underlying date/time is different between environments.

⚠️ And Then You Called .toLocaleString('en-US', { timeZone })

Which formats the Date (already off!) in the timezone you provide, like America/Chicago. But at that point, the date/time is already wrong.

✅ The Fix Recap

Since your environments use different system timezones, and the JS Date constructor is sensitive to system timezone, the only safe approach is:

• Do not create dates using strings like '2025-04-30T10:00:00'

• Instead, use a timezone-aware formatter (Intl.DateTimeFormat) to interpret your date + time in the desired time zone, then manually create a UTC Date

GPT 2nd attempt

🕒 Native Timezone Conversion in JavaScript (Environment-Safe)

This snippet provides a reliable, consistent, and environment-independent way to convert a date + time in a specific IANA time zone (e.g., America/Chicago) into an accurate UTC ISO string using only native JavaScript.

✅ Problem

The built-in Date constructor interprets strings like '2025-04-30T10:00:00' in the system's local time zone, which leads to inconsistent behavior between environments (e.g., dev vs. production containers with different system timezones).

✅ Goal

Given:

date = '2025-04-30'
time = '10:00:00'
timeZone = 'America/Chicago'

We want to produce:

2025-04-30T15:00:00.000Z

That is: convert 10:00am Chicago time to UTC consistently across environments.

✅ Solution

export function dateToISOTimezone(
  date: string,
  time: string,
  timeZone: string
) {
  // 1. Start with UTC date from input
  const utcDate = new Date(`${date}T${time}Z`)

  // 2. Use Intl.DateTimeFormat to get the local time in the desired timeZone
  const options = {
    timeZone,
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
    hour12: false
  } as const

  const formatter = new Intl.DateTimeFormat('en-US', options)
  const parts = formatter.formatToParts(utcDate)

  const get = (type: string) => Number(parts.find(p => p.type === type)?.value)

  const tzDate = new Date(Date.UTC(
    get('year'),
    get('month') - 1,
    get('day'),
    get('hour'),
    get('minute'),
    get('second')
  ))

  // 3. Calculate offset: tzDate - utcDate = offset (in ms)
  const offsetMs = tzDate.getTime() - utcDate.getTime()

  // 4. Apply offset to UTC date to get the correct ISO
  const shifted = new Date(utcDate.getTime() + offsetMs)

  return shifted.toISOString()
}

✅ Why This Works

• utcDate is based on UTC, no local-time ambiguity.
• Intl.DateTimeFormat(..., { timeZone }) tells you how that UTC time would look in the desired time zone.
• You derive the time zone offset from that formatting.
• Finally, you apply that offset to the original UTC date to get the correct universal time.

✅ Example Output

dateToISOTimezone('2025-04-30', '10:00:00', 'America/Chicago')
// → '2025-04-30T15:00:00.000Z'

This technique works without libraries, avoids pitfalls of local timezone interpretation, and ensures consistency across environments like local dev machines and production containers.