Document JapaneseCalendar.MinSupportedDateTime breaking change in .NET 11 Preview 1

[Copilot]

The Japanese Calendar minimum supported date changed from 1868-09-08 to 1868-10-23 in .NET 11 Preview 1 to align with Unicode CLDR data (CLDR-11375). This affects JapaneseCalendar.MinSupportedDateTime and invalidates dates in the 45-day gap.

Changes

• Created docs/core/compatibility/globalization/11.0/japanese-calendar-min-date.md

  • Documents behavioral change affecting JapaneseCalendar.MinSupportedDateTime
  • Provides migration guidance for code using dates between 1868-09-08 and 1868-10-23
  • References Unicode CLDR update for historical accuracy

• Created docs/core/compatibility/11.0.md

  • New index file for .NET 11 breaking changes

• Updated docs/core/compatibility/toc.yml

  • Added .NET 11 section with globalization subsection

Example Impact

var calendar = new JapaneseCalendar();
var date = new DateTime(1868, 9, 15); // Previously valid

// .NET 11: Throws ArgumentOutOfRangeException
// Date falls before MinSupportedDateTime (1868-10-23)
calendar.GetYear(date);

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Breaking change]: Correct the Japanese Calendar supported minimum Gregorian date</issue_title>
<issue_description>### Description

The start date of the Japanese Meiji era has been updated from 1868-09-08 to 1868-10-23 to reflect the latest Unicode CLDR (Common Locale Data Repository) data and improved historical accuracy. This date also serves as the minimum supported date in the Japanese calendar.

Version

.NET 11 Preview 1

Previous behavior

JapaneseCalendar.MinSupportedDateTime previously returned 1868-09-08. Dates between 1868-09-08 and 1868-10-23 were also considered valid and supported by JapaneseCalendar.

New behavior

JapaneseCalendar.MinSupportedDateTime returns 1868-10-23 instead. Dates between 1868-09-08 and 1868-10-23 now considered invalid and unsupported by JapaneseCalendar.

Type of breaking change

• Binary incompatible: Existing binaries might encounter a breaking change in behavior, such as failure to load or execute, and if so, require recompilation.
• Source incompatible: When recompiled using the new SDK or component or to target the new runtime, existing source code might require source changes to compile successfully.
• Behavioral change: Existing binaries might behave differently at run time.

Reason for change

To reflect the latest Unicode CLDR (Common Locale Data Repository) https://unicode-org.atlassian.net/browse/CLDR-11375 data and improved historical accuracy.

Recommended action

Update any code depending on the JapaneseCalendar.MinSupportedDateTime old value and avoid using Gregorian dates before 1868-10-23 with the JapaneseCalendar.

Feature area

Globalization

Affected APIs

JapaneseCalendar
JapaneseCalendar.MinSupportedDateTime

---

Associated WorkItem - 543457</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Breaking change]: Correct the Japanese Calendar supported minimum Gregorian date #50894

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/11.md	Breaking changes in .NET 11
docs/core/compatibility/core-libraries/11/environment-tickcount-windows-behavior.md	Environment.TickCount made consistent with Windows timeout behavior
docs/core/compatibility/globalization/11/japanese-calendar-min-date.md	"Breaking change: Japanese Calendar minimum supported date corrected"
docs/core/compatibility/toc.yml	docs/core/compatibility/toc

[Copilot]

Pull request overview

This PR documents a breaking change in .NET 11 Preview 1 where the Japanese Calendar's minimum supported date was corrected from 1868-09-08 to 1868-10-23 to align with Unicode CLDR data. The change updates the JapaneseCalendar.MinSupportedDateTime property value and invalidates dates in the 45-day gap between the old and new dates.

Key changes:

• Creates new breaking change documentation for the Japanese Calendar date correction
• Adds .NET 11 Globalization section to the breaking changes index
• Updates the table of contents to include the new breaking change entry

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/core/compatibility/globalization/11/japanese-calendar-min-date.md	New breaking change documentation describing the Japanese Calendar minimum date correction, including previous/new behavior, reason for change, and affected APIs
docs/core/compatibility/11.md	Adds Globalization section to the .NET 11 breaking changes index with entry for Japanese Calendar change
docs/core/compatibility/toc.yml	Updates table of contents to include Globalization subsection under .NET 11 with link to new Japanese Calendar article
docs/core/compatibility/core-libraries/11/environment-tickcount-windows-behavior.md	Corrects title punctuation from hyphen to colon for consistency with other breaking change documents

[BillWagner]

This LGTM. Let's