Simplify TryParseFormatO fractional-seconds handling

[danmoseley]

Simplify TryParseFormatO fractional-seconds handling

The "O" (ISO 8601 round-trip) format always has exactly 7 fractional-second digits, which matches DateTime's tick precision (TimeSpan.TicksPerSecond == 10_000_000). The integer formed by those seven digits is therefore exactly the sub-second tick count, so we can pass it straight to TryAddTicks instead of routing it through a double divide / multiply / Math.Round round-trip.

The change is bit-exact: every one of the 10M possible 7-digit values produces the same tick count as before (verified by brute-force).

Tests

Existing System.Tests.DateTimeTests (1378 tests) and System.Tests.DateTimeOffsetTests (737 tests, 2 platform-skipped) pass against a CoreLib built with the change.

Microbenchmark

Side benefit: a small (~6%) speedup on Perf_DateTime.ParseO on R2R'd CoreLib.

Benchmarks used (unchanged, from dotnet/performance):

• Perf_DateTime.ParseO — target
• Perf_DateTime.ParseR — unchanged control

Run with BenchmarkDotNet Job.Default, same-process A/B (both --coreRuns passed in one invocation so process-startup, ASLR, and thermal effects don't bias the comparison), affinity-pinned to a single P-core:

dotnet run -c Release -f net11.0 --project src/benchmarks/micro/MicroBenchmarks.csproj -- \
  --filter "System.Tests.Perf_DateTime.ParseO" "System.Tests.Perf_DateTime.ParseR" \
  --coreRun <baseline-testhost>\CoreRun.exe <patched-testhost>\CoreRun.exe \
  --affinity 1

Environment: Windows 11, Intel Core i9-14900K (P-core 0 pinned), .NET 11.0 R2R'd System.Private.CoreLib (Release), BenchmarkDotNet v0.16.0-nightly.

Method	Job	Mean	Error	StdDev	Ratio	RatioSD
ParseR	baseline	11.42 ns	0.168 ns	0.149 ns	1.00	0.00
ParseR	patched	11.41 ns	0.163 ns	0.144 ns	1.00	0.02
ParseO	baseline	11.87 ns	0.198 ns	0.175 ns	1.00	0.00
ParseO	patched	11.14 ns	0.125 ns	0.111 ns	0.94	0.02

ParseR (control) is identical between the two binaries, confirming the harness is stable; ParseO improves ~6% (~0.73 ns/call).

Note

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[danmoseley]

I was testing copilot and had it do something useful 😄

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[tarekgh]

LGTM, Thanks!