C# Add 1 day in specific TimeZone to DateTimeOffset

Question

I have an instance of DateTimeOffset and I need to add 1 day to it in specific TimeZone (W. Europe Standard Time) taking into account daylight savi

Answer 1

TimeZoneInfo makes this reasonably simple - just add a day to the DateTime part of the value, check whether the result is skipped or ambiguous, and if not, ask the zone for the UTC offset. Here's a complete example showing all the different possibilities:

using System;
using System.Globalization;

using static System.FormattableString;

class Program
{
    static void Main()
    {        
        // Stay in winter
        Test("2017-01-22T15:00:00+01:00");
        // Skipped time during transition
        Test("2017-03-25T02:30:00+01:00");
        // Offset change to summer
        Test("2017-03-25T15:00:00+01:00");
        // Stay in summer
        Test("2017-06-22T15:00:00+02:00");
        // Ambiguous time during transition
        Test("2017-10-28T02:30:00+02:00");
        // Offset change back to winter
        Test("2017-10-28T15:00:00+02:00");
        // Stay in winter
        Test("2017-12-22T15:00:00+01:00");
    }

    static void Test(string startText)
    {
        var zone = TimeZoneInfo.FindSystemTimeZoneById("W. Europe Standard Time");
        var start = DateTimeOffset.ParseExact(
            startText, "yyyy-MM-dd'T'HH:mm:ssK", CultureInfo.InvariantCulture);
        try
        {
            var end = AddOneDay(start, zone);
            Console.WriteLine(Invariant($"{startText} => {end:yyyy-MM-dd'T'HH:mm:ssK}"));
        }
        catch (Exception e)
        {
            Console.WriteLine($"{startText} => {e.Message}");
        }
    }

    static DateTimeOffset AddOneDay(DateTimeOffset start, TimeZoneInfo zone)
    {
        var newLocal = start.DateTime.AddDays(1);
        // TODO: Use a better exception type :)
        if (zone.IsAmbiguousTime(newLocal))
        {
            throw new Exception("Ambiguous");
        }
        if (zone.IsInvalidTime(newLocal))
        {
            throw new Exception("Skipped");
        }
        return new DateTimeOffset(newLocal, zone.GetUtcOffset(newLocal));
    }

}