如何计算 C# 中两个日期之间我时区的实际经过时间？

Question

How do I calculate the actual elapsed time between two DateTime / DateTimeOffset , both in my timezone, taking daylight saving time, leap years, etc, into account?

For example, the following code comparing the difference between 00:00 and 05:00 of March 29th 2020 will output a timespan of 6 hours (05:00 becomes 06:00 after the conversion to local time, since daylight saving time starts 02:00 in my timezone, changing clocks forward from 02:00 to 03:00).

var timezone = TimeZoneInfo.Local;

var start = new DateTimeOffset(2020, 03, 29, 00, 00, 00, timezone.BaseUtcOffset).LocalDateTime;
var end   = new DateTimeOffset(2020, 03, 29, 05, 00, 00, timezone.BaseUtcOffset).LocalDateTime;

var diff1 = end.Subtract(start);
var diff2 = end - start;

Console.WriteLine(diff1); // 06:00:00
Console.WriteLine(diff2); // 06:00:00

I'm trying to get an output of 4 hours in the example above since that is the elapsed time between 00:00 and 05:00.

00:00 -> 01:00 (+ 1 hr)
01:00 -> 02:00 (+ 1 hr)
02:00 -> 03:00 (0)
03:00 -> 04:00 (+ 1 hr)
04:00 -> 05:00 (+ 1 hr)

= 4 hrs

I'm clearly missing something here. I've read Comparisons and arithmetic operations with DateTimeOffset values but can't wrap my head around how to do this calculation.

I am aware that NodaTime can probably achieve this, but surely there's a way to do this without external libraries?

Answer 1

As Panagiotis Kanavos pointed out,

So the offset is wrong. It's no longer the BaseUtcOffset, it's a different one

I found TimeZoneInfo.GetUtcOffset and used that. I think it does what I want? Timezones are confusing, but at least I get my output of 4 hours now.

var timezone = TimeZoneInfo.Local;

var startTime = new DateTime(2020, 03, 29, 00, 00, 00);
var endTime   = new DateTime(2020, 03, 29, 05, 00, 00);

var startTimeOffset = timezone.GetUtcOffset(startTime);
var endTimeOffset   = timezone.GetUtcOffset(endTime);

var start = new DateTimeOffset(2020, 03, 29, 00, 00, 00, startTimeOffset);
var end   = new DateTimeOffset(2020, 03, 29, 05, 00, 00, endTimeOffset);

var diff1 = end.Subtract(start);
var diff2 = end - start;

Console.WriteLine(diff1); // 04:00:00
Console.WriteLine(diff2); // 04:00:00

Answer 2

Your answer is fine, and is the correct approach when the offset is not provided with if the time zone, assuming it can be any arbitrary time zone.

However, if you are only working with the local system time zone, you can simplify the code as follows:

var start = new DateTimeOffset(new DateTime(2020, 3, 29, 0, 0, 0, DateTimeKind.Local));
var end   = new DateTimeOffset(new DateTime(2020, 3, 29, 5, 0, 0, DateTimeKind.Local));
TimeSpan diff = end - start;

This works because the .Kind property of a DateTime is evaluated when passed into the DateTimeOffset constructor that takes a single DateTime parameter. If it's DateTimeKind.Local or DateTimeKind.Utc , then the offset is set correctly. It's only in the case of DateTimeKind.Unspecified that the offset could be incorrect (because it will assume local, even if that's not the case). See the remarks in the docs here.