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Fix DateTime parsing when AM designator prefix PM designator #123024

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Merged
tarekgh merged 3 commits into from
Jan 9, 2026
Merged

Fix DateTime parsing when AM designator prefix PM designator #123024

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ffce9e7
Fix DateTime parsing when AM designator prefix PM designator
tarekgh Jan 9, 2026
1d689bc
Update src/libraries/System.Private.CoreLib/src/System/Globalization/…
tarekgh Jan 9, 2026
286721b
Address the feedback
tarekgh Jan 9, 2026
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28 changes: 20 additions & 8 deletions src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3744,24 +3744,36 @@ private static bool MatchTimeMark(ref __DTString str, DateTimeFormatInfo dtfi, s

if (str.GetNext())
{
string searchStr = dtfi.AMDesignator;
if (searchStr.Length > 0)
string amDesignator = dtfi.AMDesignator;
string pmDesignator;

if (amDesignator.Length > 0)
{
if (str.MatchSpecifiedWord(searchStr))
if (str.MatchSpecifiedWord(amDesignator))
{
pmDesignator = dtfi.PMDesignator;
if (pmDesignator.StartsWith(amDesignator, StringComparison.Ordinal) && str.MatchSpecifiedWord(pmDesignator))
{
// AM designator is a prefix of PM designator and we have matched PM designator. Use longer match.
str.Index += (pmDesignator.Length - 1);
result = TM.PM;
return true;
}

// Found an AM timemark with length > 0.
str.Index += (searchStr.Length - 1);
str.Index += (amDesignator.Length - 1);
result = TM.AM;
return true;
}
}
searchStr = dtfi.PMDesignator;
if (searchStr.Length > 0)

pmDesignator = dtfi.PMDesignator;
if (pmDesignator.Length > 0)
{
if (str.MatchSpecifiedWord(searchStr))
if (str.MatchSpecifiedWord(pmDesignator))
{
// Found a PM timemark with length > 0.
str.Index += (searchStr.Length - 1);
str.Index += (pmDesignator.Length - 1);
result = TM.PM;
return true;
}
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17 changes: 17 additions & 0 deletions src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3085,5 +3085,22 @@ public void TestDateTimeCalculationPrecision(double value, DateTime initialValue
break;
}
}

[Fact]
public void TestParsingWithAmPrefixPm()
{
var culture = new CultureInfo("en-US");
DateTime dt = new DateTime(2023, 4, 17, 14, 30, 0);

// AM designator is a prefix of PM designator
culture.DateTimeFormat.AMDesignator = "aM";
culture.DateTimeFormat.PMDesignator = "aMP";

string formatted = dt.ToString("hh:mm tt", culture);
Assert.Equal("02:30 aMP", formatted);

DateTime parsedDateTime = DateTime.ParseExact(formatted, "hh:mm tt", culture);
Assert.Equal(dt.TimeOfDay, parsedDateTime.TimeOfDay);
}
}
}
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