Moment.js Locale Override Mystery - AM/PM Parsing Gone Wrong!"

[shakeelkoper]

Describe the bug
The moment().locale() function is not able to override the locale from 'ja' to 'en' for a specific instance. As a result, it's not able to correctly parse the "AM/PM" part of a UTC date-time string. ex: '5/11/2024 3:00:31 PM'

To Reproduce
Steps to reproduce the behavior:

1. Set the global locale to 'ja' using moment.locale('ja').
2. Try to parse a date-time string in English format with the "AM/PM" part, using moment().locale('en').
   ex:moment("5/11/2024 3:00:31 PM", "M/D/YYYY h:mm:ss a").locale('en').utcOffset(0, true).unix()
3. The "AM/PM" part is not parsed correctly, as indicated by unusedInput:["PM"] and unusedTokens: ["a"].

Expected behavior
The moment().locale('en') function should override the global locale for the specific instance, allowing the "AM/PM" part of the date-time string to be parsed correctly.
1715439631 -> Saturday, 11 May 2024 15:00:31

Wrong behavior
1715396431 -> Saturday, 11 May 2024 03:00:31

Desktop (please complete the following information):

• OS: macOS 14.4.1 (23E224)
• Node.js version: v16.20.2
• Moment.js version: 2.30.1

Moment-specific environment

• The time zone setting of the machine the code is running on: [e.g. JST]
• The time and date at which the code was run: [e.g. '5/10/2024 8:58:17 PM']
• Other libraries in use: moment-duration-format 2.3.2, moment-timezone 0.5.34

Please run the following code in your environment and include the output:

console.log((new Date()).toString()) //'Thu May 16 2024 01:28:31 GMT+0530 (India Standard Time)'
console.log((new Date()).toLocaleString()) //'16/05/2024, 01:28:47'
console.log((new Date()).getTimezoneOffset()) //-330
console.log(process.version)
console.log(moment.version) //'2.30.1'

[gilmoreorless]

moment("5/11/2024 3:00:31 PM", "M/D/YYYY h:mm:ss a").locale('en').utcOffset(0, true).unix()

In your code example, you're only calling .locale('en') after the parsing has already happened. That snippet is not trying to parse the string in en locale; it's trying to parse the string in the global locale then use the en locale afterwards.

[gilmoreorless]

There are already examples in the parsing documentation:

As of version 2.0.0, a locale key can be passed as the third parameter to moment() and moment.utc().

moment('2012 juillet', 'YYYY MMM', 'fr');
moment('2012 July',    'YYYY MMM', 'en');
moment('2012 July',    'YYYY MMM', ['qj', 'en']);