DateTime Comparison fails with different DateTimeKind

[AdamRadicsTalison]

Description

When comparing UTC vs local time, it seems the offset is not applied before comparison.

Reproduction Steps

I am in the UTC+8 time zone.

// 9.20am
DateTime laterUTC = new DateTime(2023, 06, 26, 01, 20, 00, DateTimeKind.Utc);

// 08:50am
DateTime earlierLocal = new DateTime(2023, 06, 26, 08, 50, 00, DateTimeKind.Local);

laterUTC.ToString("O").Dump();
earlierLocal.ToString("O").Dump();

($"Later UTC < Earlier Local: {(laterUTC < earlierLocal)}").Dump();

($"Later UTC < Earlier UTC: {(laterUTC < earlierLocal.ToUniversalTime())}").Dump();

($"Later Local < Earlier Local: {(laterUTC.ToLocalTime() < earlierLocal)}").Dump();

Expected behavior

2023-06-26T01:20:00.0000000Z
2023-06-26T08:50:00.0000000+08:00
Later UTC < Earlier Local: False
Later UTC < Earlier UTC: False
Later Local < Earlier Local: False

Actual behavior

2023-06-26T01:20:00.0000000Z
2023-06-26T08:50:00.0000000+08:00
Later UTC < Earlier Local: True
Later UTC < Earlier UTC: False
Later Local < Earlier Local: False

Regression?

No response

Known Workarounds

Convert kind before comparison.

Configuration

No response

Other information

No response

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Issue Details

---

Description

When comparing UTC vs local time, it seems the offset is not applied before comparison.

Reproduction Steps

I am in the UTC+8 time zone.

// 9.20am
DateTime laterUTC = new DateTime(2023, 06, 26, 01, 20, 00, DateTimeKind.Utc);

// 08:50am
DateTime earlierLocal = new DateTime(2023, 06, 26, 08, 50, 00, DateTimeKind.Local);

laterUTC.ToString("O").Dump();
earlierLocal.ToString("O").Dump();

($"Later UTC < Earlier Local: {(laterUTC < earlierLocal)}").Dump();

($"Later UTC < Earlier UTC: {(laterUTC < earlierLocal.ToUniversalTime())}").Dump();

($"Later Local < Earlier Local: {(laterUTC.ToLocalTime() < earlierLocal)}").Dump();

Expected behavior

2023-06-26T01:20:00.0000000Z
2023-06-26T08:50:00.0000000+08:00
Later UTC < Earlier Local: False
Later UTC < Earlier UTC: False
Later Local < Earlier Local: False

Actual behavior

2023-06-26T01:20:00.0000000Z
2023-06-26T08:50:00.0000000+08:00
Later UTC < Earlier Local: True
Later UTC < Earlier UTC: False
Later Local < Earlier Local: False

Regression?

No response

Known Workarounds

Convert kind before comparison.

Configuration

No response

Other information

No response

Author:	AdamRadicsTalison
Assignees:	-
Labels:	area-System.DateTime
Milestone:	-

[huoyaoyuan]

I'll recommend using DateTimeOffset everywhere to get consistent result. Personally I ban the usage of DateTime at all.

[Clockwork-Muse]

From the documentation:

To determine the relationship of t1 to t2, the Compare method compares the Ticks property of t1 and t2 but ignores their Kind property. Before comparing DateTime objects, ensure that the objects represent times in the same time zone.

So: by design.

The reason for this is that DateTimeKind was introduced after DateTime, and changing the behavior here would have been a massive breaking change.

[tarekgh]

@Clockwork-Muse is correct. Also, it will not be reliable to allow this comparison because it wouldn't make sense when having Unspecified Kind. Or even local Kind can be serialized to another machine which has different zone. @huoyaoyuan suggestion is good which try use DateTimeOffset in your case.

[Clockwork-Muse]

(Note that this is part of why I revile DateTimeKind, because it makes it extremely difficult to work with DateTime at all)