keepLocalTime applies day light savings

Describe the bug I have recently begun using Luxon and I’m really enjoying it so far. Apologies if I have misunderstood the purpose of the keepLocalTime option. As I understood it, it should return a local time with a different offset but the same date and time. But what I am seeing is day light savings being applied.

To Reproduce

const ny = DateTime.fromISO('2021-03-28T23:59:00.000Z')
  .setZone('America/New_York', { keepLocalTime: true })

ny.toFormat('DDDD HH:mm')
// "Monday, 29 March 2021 00:59"  (Actual - Unexpected)
// "Monday, 28 March 2021 23:59" (Expected)

const ny2 = luxon.DateTime.fromISO('2021-02-28T23:59:00.000Z')
  .setZone('America/New_York', { keepLocalTime: true })

ny2.toFormat('DDDD HH:mm')
// "Sunday, 28 February 2021 23:59" (Expected)

Actual vs Expected behavior I would have expected ny.toFormat('DDDD HH:mm') to have given me "Sunday, 28 March 2021 23:59" but instead it appears to apply DST and prints "Monday, 29 March 2021 00:59"

Desktop:

• OS: Linux Kubuntu
• Browser: Firefox 94.0
• Luxon version: 2.1.1
• Your timezone: Europe/London

Again apologies if I have misunderstood the purpose of the keepLocalTime and this is actually intentional behaviour.

[End-S]

That makes a lot of sense and will help me resolve my issue. Thank you for taking the time to explain it.

[icambron]

Oh, I see. No, this is not a bug. What’s happening is this:

// parse a UTC date string and convert to system time (which for you is BST)
const aLocalDt =  DateTime.fromISO('2021-03-28T23:59:00.000Z');

aLocalDt.toISO() // pretty sure this produces 00:59+1:00 for you

// convert to US EDT but holding the 00:59 constant
aLocalDt.setZone("America/New_York", { keepLocalTime: true });

In other words, the DST shift comes from your local zone before the setZone() has ever been called.

What you meant was to keep the offset from the string. You can do that one of a few ways:

1. Explicitly keep the DateTime in UTC, like DateTime.fromISO(s, { zone: "UTC"})
2. Accept the offset from the string as not just the zone to interpret the string in, but also keep the resulting DateTime in: DateTime.fromISO(s, { setZone: true })
3. Switch to UTC explicitly: DateTime.fromISO(s).setZone("utc").setZone("America/New_York", { keepLocalTime: true })
4. Run your whole environment in UTC instead of Europe/London