Document JapaneseCalendar.MinSupportedDateTime breaking change in .NET 11 Preview 1 (#50920)

Author: Copilot

docs/core/compatibility/11.md

@@ -19,3 +19,9 @@ If you're migrating an app to .NET 11, the breaking changes listed here might af
 | Title                                                             | Type of change    |
 |-------------------------------------------------------------------|-------------------|
 | [Environment.TickCount made consistent with Windows timeout behavior](core-libraries/11/environment-tickcount-windows-behavior.md) | Behavioral change |
+
+## Globalization
+
+| Title | Type of change    |
+|-------|-------------------|
+| [Japanese Calendar minimum supported date corrected](globalization/11/japanese-calendar-min-date.md) | Behavioral change |

docs/core/compatibility/core-libraries/11/environment-tickcount-windows-behavior.md

@@ -1,5 +1,5 @@
 ---
-title: "Breaking change - Environment.TickCount made consistent with Windows timeout behavior"
+title: "Breaking change: Environment.TickCount made consistent with Windows timeout behavior"
 description: "Learn about the breaking change in .NET 11 where Environment.TickCount and Environment.TickCount64 on Windows now exclude sleep and hibernation time, consistent with OS wait APIs."
 ms.date: 01/07/2026
 ai-usage: ai-assisted

docs/core/compatibility/globalization/11/japanese-calendar-min-date.md

@@ -0,0 +1,39 @@
+---
+title: "Breaking change: Japanese Calendar minimum supported date corrected"
+description: "Learn about the breaking change in .NET 11 where the Japanese Calendar minimum supported date was corrected from 1868-09-08 to 1868-10-23."
+ms.date: 01/07/2026
+ai-usage: ai-assisted
+---
+
+# Japanese Calendar minimum supported date corrected
+
+The start date of the Japanese Meiji era has been updated from `1868-09-08` to `1868-10-23` to reflect the latest Unicode Common Locale Data Repository (CLDR) data and improved historical accuracy. This date also serves as the minimum supported date in the Japanese calendar.
+
+## Version introduced
+
+.NET 11 Preview 1
+
+## Previous behavior
+
+Previously, <xref:System.Globalization.JapaneseCalendar.MinSupportedDateTime?displayProperty=nameWithType> returned `1868-09-08`. <xref:System.Globalization.JapaneseCalendar> accepted dates between `1868-09-08` and `1868-10-23` as valid.
+
+## New behavior
+
+Starting in .NET 11, <xref:System.Globalization.JapaneseCalendar.MinSupportedDateTime?displayProperty=nameWithType> returns `1868-10-23` instead. <xref:System.Globalization.JapaneseCalendar> now rejects dates between `1868-09-08` and `1868-10-23` as invalid.
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The change was made to reflect the latest Unicode CLDR data ([CLDR-11375](https://unicode-org.atlassian.net/browse/CLDR-11375)) and improved historical accuracy regarding the start of the Meiji era in Japan.
+
+## Recommended action
+
+Update any code that depends on the old value of <xref:System.Globalization.JapaneseCalendar.MinSupportedDateTime?displayProperty=nameWithType>. Avoid using Gregorian dates before `1868-10-23` with the <xref:System.Globalization.JapaneseCalendar>.
+
+## Affected APIs
+
+- <xref:System.Globalization.JapaneseCalendar?displayProperty=fullName>
+- <xref:System.Globalization.JapaneseCalendar.MinSupportedDateTime?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -12,6 +12,10 @@ items:
             items:
               - name: Environment.TickCount made consistent with Windows timeout behavior
                 href: core-libraries/11/environment-tickcount-windows-behavior.md
+          - name: Globalization
+            items:
+              - name: Japanese Calendar minimum supported date corrected
+                href: globalization/11/japanese-calendar-min-date.md
       - name: .NET 10
         items:
           - name: Overview