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Simplify TryParseFormatO fractional-seconds handling #129006

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Simplify TryParseFormatO fractional-seconds handling #129006

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10 changes: 6 additions & 4 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 @@ -5171,7 +5171,9 @@ private static bool TryParseFormatO(ReadOnlySpan<char> source, scoped ref DateTi
second = (int)(s1 * 10 + s2);
}

double fraction;
// The seven fractional digits are sub-second ticks (1 tick = 100 ns = 10^-7 s),
// matching TimeSpan.TicksPerSecond, so we can use them directly without floating-point math.
int fractionTicks;
{
uint f1 = (uint)(source[20] - '0');
uint f2 = (uint)(source[21] - '0');
Expand All @@ -5187,12 +5189,12 @@ private static bool TryParseFormatO(ReadOnlySpan<char> source, scoped ref DateTi
return false;
}

fraction = (f1 * 1000000 + f2 * 100000 + f3 * 10000 + f4 * 1000 + f5 * 100 + f6 * 10 + f7) / 10000000.0;
fractionTicks = (int)(f1 * 1000000 + f2 * 100000 + f3 * 10000 + f4 * 1000 + f5 * 100 + f6 * 10 + f7);
}

// Per ISO 8601, 24:00:00 represents the end of a calendar day
// (the same instant as the next day's 00:00:00), but only when minute, second, and fraction are all zero
if (hour == 24 && (minute != 0 || second != 0 || fraction != 0))
if (hour == 24 && (minute != 0 || second != 0 || fractionTicks != 0))
{
result.SetBadDateTimeFailure();
return false;
Expand All @@ -5204,7 +5206,7 @@ private static bool TryParseFormatO(ReadOnlySpan<char> source, scoped ref DateTi
return false;
}

if (!dateTime.TryAddTicks((long)Math.Round(fraction * TimeSpan.TicksPerSecond), out result.parsedDate))
if (!dateTime.TryAddTicks(fractionTicks, out result.parsedDate))
{
result.SetBadDateTimeFailure();
return false;
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