# What will happen to Julia's DateTime type

**URL:** <https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048>\
**Category:** General Usage\
**Tags:** datetime\
**Created:** [October 16, 2026, 6:00am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048 "2026-10-16T06:00:41Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![fabiangans](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fabiangans/32/2624_2.png) [@fabiangans](https://discourse.julialang.org/u/fabiangans)\
**Post date:** [October 16, 2026, 6:00am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048/1 "2026-10-16T06:00:41Z")

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Do we need to redefine Julias DateTime type, now that UTC will not follow UT1 anymore?

> **[Scientists just voted to redefine how the world keeps time](https://www.scientificamerican.com/article/scientists-vote-to-end-leap-seconds-redefining-how-the-world-keeps-time/)**
>
> A new system, called the “leap hour,” will allow Earth’s rotation and official time to fall out of alignment by a full hour

I guess this will be very tricky to solve. When UTC and UT are beginning to diverge in the future, we probably do not want our default DateTime to be in UT. Of course there is probably no need for immediate action, since it will take decades for both to diverge significantly, but I would be interested if there are already ideas out how to solve potential issues.

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [October 16, 2026, 6:12am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048/2 "2026-10-16T06:12:30Z")

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`DateTime` already doesn’t deal with leap seconds in UTC, so having leap hours (which `DateTime` will also ignore) seems even better because they are so infrequent.

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**Author:** ![fabiangans](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fabiangans/32/2624_2.png) [@fabiangans](https://discourse.julialang.org/u/fabiangans)\
**Post date:** [October 16, 2026, 9:44am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048/3 "2026-10-16T09:44:52Z")

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Well I think the problem is that DateTime currently follows UT which keeps the clocks synchronized with solar noon/midnight by defining a second through the duration of the Earth’s rotation. This way DateTime times were almost in sync with UTC (plus-minus one second). However, in the future UTC, based on a fixed second will slowly drift away from UT so that solar time will not match UTC anymore and therefore Julia’s DateTime and UTC will also start diverging because UTC stops correcting through leap seconds.

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**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [October 16, 2026, 11:11am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048/4 "2026-10-16T11:11:25Z")

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> [@fabiangans](#):
>
> DateTime currently follows UT which keeps the clocks synchronized with solar noon/midnight by defining a second through the duration of the Earth’s rotation.

If that is what the doc says, it seems it has not been fixed since [this thread](https://discourse.julialang.org/t/universal-time-vs-utc-time-in-dates/103204). The understanding there seems to be that a docs fix saying “UTC-like, no leap seconds, not UT1” would be reasonable. The type itself needs no change.

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<div class="post-metadata">

**Author:** ![fabiangans](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fabiangans/32/2624_2.png) [@fabiangans](https://discourse.julialang.org/u/fabiangans)\
**Post date:** [October 16, 2026, 11:16am UTC](https://discourse.julialang.org/t/what-will-happen-to-julias-datetime-type/140048/5 "2026-10-16T11:16:19Z")

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This is what I took from the docs:

> Both [`Date`](https://docs.julialang.org/en/v1/stdlib/Dates/#Dates.Date) and [`DateTime`](https://docs.julialang.org/en/v1/stdlib/Dates/#Dates.DateTime) are basically immutable [`Int64`](https://docs.julialang.org/en/v1/base/numbers/#Core.Int64) wrappers. The single `instant` field of either type is actually a `UTInstant{P}` type, which represents a continuously increasing machine timeline based on the UT second [[1]](https://docs.julialang.org/en/v1/stdlib/Dates/#footnote-1).

I think it is not super-clear but my takeaway was that it represents UT.