Fix DateTime parsing when AM designator prefix PM designator#123024
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tarekgh merged 3 commits intodotnet:mainfrom Jan 9, 2026
Merged
Fix DateTime parsing when AM designator prefix PM designator#123024tarekgh merged 3 commits intodotnet:mainfrom
tarekgh merged 3 commits intodotnet:mainfrom
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Pull request overview
This PR fixes a bug in DateTime parsing where the AM designator is a prefix of the PM designator (e.g., "aM" and "aMP"). The fix ensures that when both designators match, the longer PM designator is used instead of incorrectly matching the shorter AM designator.
Key changes:
- Adds logic to check if PM designator starts with AM designator and prefers the longer match
- Includes a comprehensive test case that verifies both formatting and parsing with prefix scenario
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs | Adds logic in MatchTimeMark to handle the case where AM designator is a prefix of PM designator by checking for and preferring the longer PM match |
| src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs | Adds test case TestParsingWithAmPrefixPm that validates DateTime parsing when AM designator ("aM") is a prefix of PM designator ("aMP") |
src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
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stephentoub
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src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
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Tagging subscribers to this area: @dotnet/area-system-datetime |
…DateTimeParse.cs Co-authored-by: Copilot <[email protected]>
stephentoub
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src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
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Likely makes sense to merge this PR. My PR is waiting on networking related changes. You can rewrite history on the other branch if you'd like to remove your change to avoid merge conflicts. |
jeffhandley
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/ba-g failures are unrelated. |
This was referenced Jan 13, 2026
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CLDR globalization data updates changed the
ak-GHAM and PM designators toANandANW, respectively. The new values make the AM designator a prefix of the PM designator, which causes issues during date/time parsing: a value may be parsed as the AM designator even when the input contains the PM designator. The fix is to always attempt to parse the longest matching value first, and fall back to the shorter one if needed.