TimeZone Helper

Tz.cs

class Tz
{
    /// <summary>
    /// Maintain date time values from UTC DateTime but change her offset for espefict time zone.
    /// </summary>
    /// <param name="localDate">DateTime in UTC</param>
    /// <param name="timeZoneId">IANA time zone format</param>
    /// <returns></returns>
    public static DateTime ShiftUtcDateTimeToTimeZoneOffSet(DateTime localDate, string timeZoneId)
    {
        if (localDate.Kind != DateTimeKind.Utc) throw new Exception($"Can not convert to {timeZoneId} because {localDate} is not set as UTC kind.");

        if (string.IsNullOrWhiteSpace(timeZoneId))
        {
            return localDate;
        }

        localDate = DateTime.SpecifyKind(localDate, DateTimeKind.Unspecified);

        TimeZoneInfo tz;
        if (System.Environment.OSVersion.Platform == PlatformID.Win32NT)
        {
            tz = TimeZoneInfo.FindSystemTimeZoneById(TZConvert.IanaToWindows(timeZoneId));
        }
        else
        {
            tz = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);
        }

        var destinationTimeZone = TimeZoneInfo.FindSystemTimeZoneById(TZConvert.IanaToWindows("UTC"));
        if (tz.IsInvalidTime(localDate))
        {
            if (!tz.IsInvalidTime(localDate.AddHours(1)))
            {
                localDate = localDate.AddHours(1);
            }
            else if (!tz.IsInvalidTime(localDate.AddHours(-1)))
            {
                localDate = localDate.AddHours(-1);
            }
        }
        return TimeZoneInfo.ConvertTime(localDate, tz, destinationTimeZone).ToUniversalTime();

    }

    public static DateTime FromLocal(DateTime localDate, string timeZoneId)
    {
        if (localDate.Kind == DateTimeKind.Local)
        {
            return localDate.ToUniversalTime();
        }

        TimeZoneInfo tz;
        if (System.Environment.OSVersion.Platform == PlatformID.Win32NT)
        {
            tz = TimeZoneInfo.FindSystemTimeZoneById(TZConvert.IanaToWindows(timeZoneId));
        }
        else
        {
            tz = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);
        }

        return TimeZoneInfo.ConvertTimeToUtc(localDate, tz);
    }
}