Bug: Logger - Invalid ISO 8601 timestamp (`T24:00:00`) when TZ is set to any non-UTC value

[ogis-tkash]

Expected Behavior

When the TZ environment variable is set to any value other than UTC (for example, ":/etc/localtime", "Europe/London", etc.), the logger should output timestamps in valid ISO 8601 format. Specifically, the hour portion should never be 24 (e.g., 2025-05-09T24:00:33 is invalid ISO 8601); it should be to 00, according to ISO 8601.

Current Behavior

When running AWS Lambda Powertools for TypeScript with TZ set to any non-UTC value, the logger may emit timestamps such as 2025-05-09T24:00:33, which is not a valid ISO 8601 value. This seems to occur when logging events exactly at midnight, likely due to the use of Intl.DateTimeFormat and how it interprets some timezones.

Code snippet

see Steps to Reproduce.

Steps to Reproduce

1. Deploy a Lambda function using Powertools for AWS Lambda (TypeScript)
2. Set the environment variable TZ to any value other than UTC (such as :/etc/localtime, Europe/London, etc.)
3. Trigger a log event at exactly midnight (00:00:33)
4. Observe the generated timestamp in the logs; it may appear as T24:00:33

Possible Solution

For example, you could use a Unicode extension for your locale to change the hour cycle from 24 (Hour system using 1–24) to 23 (Hour system using 0–23).
see Unicode extension

powertools-lambda-typescript/packages/logger/src/formatter/LogFormatter.ts

Line 202 in 0157499

return new Intl.DateTimeFormat('en', {

    return new Intl.DateTimeFormat('en', {
      year: 'numeric',
      month: twoDigitFormatOption,
      day: twoDigitFormatOption,
      hour: twoDigitFormatOption,
      minute: twoDigitFormatOption,
      second: twoDigitFormatOption,
      hour12: false,
      timeZone: validTimeZone,
    });

↓↓↓

    return new Intl.DateTimeFormat('en-u-hc-h23', {
      year: 'numeric',
      month: twoDigitFormatOption,
      day: twoDigitFormatOption,
      hour: twoDigitFormatOption,
      minute: twoDigitFormatOption,
      second: twoDigitFormatOption,
      hour12: false,
      timeZone: validTimeZone,
    });

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

20.x

Packaging format used

npm

Execution logs

[boring-cyborg]

Thanks for opening your first issue here! We'll come back to you as soon as we can.
In the meantime, check out the #typescript channel on our Powertools for AWS Lambda Discord: Invite link

[dreamorosi]

Hi, thank you for opening this issue.

I tried to reproduce the issue using this code:

import { Logger } from '@aws-lambda-powertools/logger';

const logger = new Logger({
  serviceName: 'tz-test'
});

export const handler = async () => {
  logger.info('Hello')
}

and setting a TZ (in my example America/Anchorage since it's Midnight relative to where I am) and I got a timezone with the 00 hour rather than 24 like in the report using Node.js 22:

while I did get an incorrect timestamp when using Node.js 20:

and using Node.js 18:

Given that we are seeing a different behavior across different runtimes, before confirming the bug I'd like to reach out to the AWS Lambda product team and get their feedback to understand if this is something expected or not.

In the meantime, if your use case allows it, and you require the correct value - you could consider upgrading the runtime Node.js 22 - which appears to generate the correct format.

[ogis-tkash]

@dreamorosi

Thank you very much for your prompt investigation and detailed response!

Unfortunately, I'm currently unable to upgrade to Node.js 22 in my environment. As a workaround, I’ve implemented a custom log formatter and temporarily addressed the issue using the approach described under “Possible Solution”.

So while I’m not in urgent need of an official fix, I do plan to remove my workaround once Powertools provides a built-in solution. I appreciate your continued support!