isSame does not work correctly with time zones

[bdwain]

Describe the bug I passed the same dayjs object to both isSame parameters (with 'day' units) and got false.

Expected behavior The same object should be the same day.

Information

• Day.js Version Whatever is running on the documentation site
• OS: latest Mac
• Browser Chrome 134
• Time zone: Central Daylight Time

[bdwain]

i've seen a few issues where people talk about isSame not working correctly, but they all seem to be for cases where you pass in a string. i'm passing in objects here that have explicit time zones.it seems like those time zones are being discarded on one of them, but not both of them

[bdwain]

after some more digging, it seems like this may be potentially related to the fact that the clocks change on 3/30 in Germany? but they change at 02:00 on 3/30 and this was looking at 3/29 so im not sure why that would occur.

i think there is a related bug though where if i was to say djs1.endOf('day') using the values above, it would return UTC 21:59:59 which is not correct. it's UTC '22:59:59. endOf('day') is used internally in isSame

[kwiatkk1]

Yes, the timezone plugin overrides startOf by

• first removing the timezone info from the date,
• then running the "regular" logic TREATING THE DATE AS LOCAL,
• and finally restoring the timezone info (see: plugin/timezone/index.js#L125-L133)

So:

• we start with 2025-03-29T23:59:00+01:00 (CET).
• we have the end of the day set to 2025-03-29T23:59:59-07:00 (ISO 2025-03-30T06:59:59.999Z, so in Europe, it's CEST at this point)
• when the timezone info is added back, we get 2025-03-29T23:59:59+02:00