DateTimeOffsetAndTimezonesTests.cs

using System;
using System.Collections.Generic;
using Xunit;

namespace DaylightSavingTimeTests
{
    public class DstCalculationsTests
    {
        [Theory]
        [MemberData(nameof(DstTestCases))]
        public void ShouldCalculateTimeDifferenceAsExpected(
            DateTimeOffset from,
            DateTimeOffset until,
            TimeSpan expectedDifference,
            string description)
        {
            Assert.Equal(expectedDifference, until - from, new TimeSpanWithMillisecondPrecisionComparer(5));
        }

        public static TheoryData<DateTimeOffset, DateTimeOffset, TimeSpan, string> DstTestCases()
        {
            var utcDayBeforeGermanDst = new DateTimeOffset(2020, 3, 28, 11, 30, 00, TimeSpan.Zero);
            var utcDayAfterGermanDst = new DateTimeOffset(2020, 3, 29, 15, 17, 00, TimeSpan.Zero);
            var utcDayBeforePolishWinterTime = new DateTimeOffset(2019, 10, 26, 13, 45, 55, TimeSpan.Zero);

            return new TheoryData<DateTimeOffset, DateTimeOffset, TimeSpan, string>
            {
                {DateTimeOffset.Now, DateTimeOffset.UtcNow, TimeSpan.FromHours(0), "UtcNow and Now"},
                {DateTimeOffset.Now, DateTimeOffset.Now.AddHours(1), TimeSpan.FromHours(1), "One hour ahead, local time"},
                {DateTimeOffset.Now, DateTimeOffset.Now.AddDays(1), TimeSpan.FromHours(24), "One day ahead, local time"},
                {
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayBeforeGermanDst, "Central Europe Standard Time"),
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayBeforeGermanDst.AddDays(1), "Central Europe Standard Time"),
                    TimeSpan.FromHours(24),
                    "One day ahead, across DST start in Germany"
                },
                {
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayAfterGermanDst, "Central Europe Standard Time"),
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayAfterGermanDst.AddDays(-2), "Central Europe Standard Time"),
                    TimeSpan.FromHours(-48),
                    "Two days back, across DST start in Germany"
                },
                {
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayBeforePolishWinterTime, "Central Europe Standard Time"),
                    TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcDayBeforePolishWinterTime.AddDays(2), "Central Europe Standard Time"),
                    TimeSpan.FromHours(48),
                    "Two days ahead, across DST end in Poland"
                },
            };
        }
    }

    public class TimeSpanWithMillisecondPrecisionComparer : IEqualityComparer<TimeSpan>
    {
        private readonly int _millisecondPrecision;

        public TimeSpanWithMillisecondPrecisionComparer(int millisecondPrecision)
        {
            this._millisecondPrecision = millisecondPrecision;
        }

        public bool Equals(TimeSpan x, TimeSpan y)
        {
            var diff = (x - y).TotalMilliseconds;
            return Math.Abs(diff) < this._millisecondPrecision;
        }

        public int GetHashCode(TimeSpan obj)
        {
            return obj.GetHashCode();
        }
    }
}