.NET datetime从字符串转换为datetime时的毫秒精度问题

Question

Hi. I am trying to convert an incoming datetime value that comes to our system in a string format. It seems that when the precision of milliseconds is higher than 7, the datetime parsing in .NET does not seem to like the value and cannot convert/parse the value. I am a bit stuck on what to do for this? My only current thought is there is a limit on the millisecond size and that anymore precision is not possible? But I want to confirm this is the case rather than assume. Example:

string candidateDateTimeString = "2017-12-08T15:14:38.123456789Z";
if (!success)
        {
            success = DateTime.TryParseExact(trayportDateTimeString, "yyyy-
MM-dd'T'HH:mm:ss.fffffffff'Z'",
                CultureInfo.InvariantCulture, dateTimeStyles, out dateTime);
        }

If I reduce the 'f' values down to just 7, then date time parsing works fine. Is there a limit? Or am I doing something obvious wrong?

Answer 1

根据自定义日期和时间格式字符串文档，7是第二个分数的最大支持位数。

Answer 2

Internally, all DateTime values are represented as the number of ticks (the number of 100-nanosecond intervals) that have elapsed since 12:00:00 midnight, January 1, 0001.

https://msdn.microsoft.com/en-us/library/system.datetime(v=vs.110).aspx

see also: https://docs.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings

Answer 3

Precision of date and time values is more complex than you might think. There are different levels of precision involved:

Precision of DateTime

DateTime stores the number of ticks since 01.01.0001 00:00 as a 64 bit value. One tick is 100 nanoseconds. Since this is the maximum precision that can be stored, it makes no sense to format to a precision higher than that. You can just add as many zeros as you need to represent a higher precision. If you need to represent shorter timespans than 100 nanoseconds, you need to use a different type, such as an Int64 with a custom tick size.

Precision of DateTime.Now

When you call DateTime.Now , you get a much lower precision than DateTime can store. The exact value depends on the system clock, but it is usually in the milliseconds range.

Precision of Stopwatch

When you measure the time with Stopwatch , depending on your system, you might get the time from a high performance clock, which more precise than the clock used for DateTime.Now , but still less than 100 nanoseconds. On a system without high performance clock, the precision is the one of the regular system clock.

Summary

Unless the value that you are parsing originates from a high precision clock (like an atomic clock), parsing it to the full precision of DateTime , makes not much practical sense. And if it comes from such a source, you need to resort to a different data type to represent the value.