Is DateTime.Now's Precision Officially Documented, and Why Is It Limited?

Understanding C# DateTime.Now: Accuracy vs. Precision

Recent unit testing highlighted unexpected behavior with DateTime.UtcNow. Consecutive calls within a short interval yielded identical results, contrary to expectations of millisecond-level granularity. This raises questions regarding the precision of DateTime.Now. While Stopwatch offers superior precision for timing, understanding DateTime.Now's limitations is crucial. Is its precision officially documented (e.g., 50ms)? Why is it less precise than most CPU clocks?

The Difference: Accuracy and Precision

It's vital to differentiate between accuracy and precision. A precise clock consistently reports time differences with a specific resolution; an accurate clock aligns consistently with an external time source.

DateTime.Now serves general-purpose date and time representation—displaying the current time, scheduling, and calculating date differences. High-precision timekeeping isn't its primary function. Most computers lack access to accurate external time references, making high precision and accuracy simultaneously unattainable for DateTime.Now.

In Summary

DateTime.Now's lower precision prevents reporting false precision. Its design prioritizes accurate date and time representation within typical system constraints. For precise timing, Stopwatch remains the preferred choice.

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