Look my eyes, tell me why???

[daolou]

dayjs:

dayjs.utc('2000-01-01T06:01:02Z').utcOffset(1, true).format() 
'2000-01-01T06:01:02+01:00'
dayjs.utc('2000-01-01T06:01:02Z').utcOffset(1, true).valueOf()
946731662000

moment:

moment.utc('2000-01-01T06:01:02Z').utcOffset(1, true).format()
'2000-01-01T06:01:02+01:00'
moment.utc('2000-01-01T06:01:02Z').utcOffset(1, true).valueOf()
946702862000

[DashaIT]

First, take off your glasses :))

[thadeucity]

Investigating further I found some other odd behaviors:

The culprit appears to be the .utcOffset() function

// Environment:
//   dayjs v1.11.13 (with UTC plugin)
//   Node v22.17.0 or Deno 2.2.11 -- Both with the exact same error
//   Windows 11 (TZ = UTC–03:00)

dayjs.utc('2000-01-01T06:01:02Z').valueOf()
946706462000 // Looks ok

dayjs.utc('2000-01-01T06:01:02Z').utcOffset(1, true).valueOf()
946695662000 // Looks odd

// Difference equals my local tz offset (-3 hours)

---

This next one is kind of scary because it breaks dayjs immutability:

const date = dayjs.utc('2000-01-01T06:01:02Z');

date.valueOf();
946706462000

date.utcOffset(1, true).valueOf();
946695662000

date.valueOf();
946695662000 // Different from the previous date.valueOf()

// Basically if `.utcOffset()` is called, it mutates the date variable 
// in a way that it renders future data manipulation unreliable

[thadeucity]

@NaturalDevCR

I believe GitHub issues are not the place to throw a copy-and-paste marketing message for your product. You should at least contribute to improving our understanding of the problem or, even better, offer a solution.

Maybe—just maybe—after that you could suggest migrating to a library you know for sure doesn’t have this issue. But even then, if it’s a small or recent problem and the library’s contributors are still looking for a fix, it’s impolite to promote your own product—especially when you haven’t contributed anything and have merely pasted a low-effort, pre-made marketing text.

I wish you luck with your endeavor, and I think you should keep maintaining your code to offer us more options for date-time manipulation. I’m just suggesting better ways to promote it.

[iamkun]

fixed in #2505