DateTimeZonesUsingNodaTime.cs

using System;
using System.Collections.Generic;
using NodaTime;
using NodaTime.Extensions;
using Xunit;

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

        public static TheoryData<ZonedDateTime, ZonedDateTime, Duration, string> DstTestCases()
        {
            var clock = SystemClock.Instance;
            var zonedClock = clock.InTzdbSystemDefaultZone();
            var utcDayBeforeGermanDst = Instant.FromDateTimeUtc(new DateTime(2020, 3, 28, 11, 30, 00, DateTimeKind.Utc));
            var utcDayAfterGermanDst = Instant.FromDateTimeUtc(new DateTime(2020, 3, 29, 15, 17, 00, DateTimeKind.Utc));
            var utcDayBeforePolishWinterTime = Instant.FromDateTimeUtc(new DateTime(2019, 10, 26, 13, 45, 55, DateTimeKind.Utc));
            var timeZoneSource = NodaTime.TimeZones.TzdbDateTimeZoneSource.Default;

            return new TheoryData<ZonedDateTime, ZonedDateTime, Duration, string>
            {
                {zonedClock.GetCurrentZonedDateTime(), clock.GetCurrentInstant().InUtc(), Duration.FromHours(0), "UtcNow and Now"},
                {zonedClock.GetCurrentZonedDateTime(), zonedClock.GetCurrentZonedDateTime().PlusHours(1), Duration.FromHours(1), "One hour ahead, local time"},
                {zonedClock.GetCurrentZonedDateTime(), zonedClock.GetCurrentZonedDateTime().PlusHours(24), Duration.FromHours(24), "One day ahead, local time"},
                {
                    new ZonedDateTime(utcDayBeforeGermanDst, timeZoneSource.ForId("Europe/Berlin")),
                    new ZonedDateTime(utcDayBeforeGermanDst.Plus(Duration.FromDays(1)), timeZoneSource.ForId("Europe/Berlin")),
                    Duration.FromHours(24),
                    "One day ahead, across DST start in Germany"
                },
                {
                    new ZonedDateTime(utcDayAfterGermanDst, timeZoneSource.ForId("Europe/Berlin")),
                    new ZonedDateTime(utcDayAfterGermanDst.Minus(Duration.FromDays(2)), timeZoneSource.ForId("Europe/Berlin")),
                    Duration.FromHours(-48),
                    "Two days back, across DST start in Germany"
                },
                {
                    new ZonedDateTime(utcDayBeforePolishWinterTime, timeZoneSource.ForId("Europe/Warsaw")),
                    new ZonedDateTime(utcDayBeforePolishWinterTime.Plus(Duration.FromDays(2)), timeZoneSource.ForId("Europe/Warsaw")),
                    Duration.FromHours(48),
                    "Two days ahead, across DST end in Poland"
                },
            };
        }
    }

    public class DurationWithMillisecondPrecisionComparer : IEqualityComparer<Duration>
    {
        private readonly int _millisecondPrecision;

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

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

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