TimeZone Conversion for NetCore - Obsolete - Use the WinRTTimeZones Nuget package for the latest

Util.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

// OBSOLETE!!! Use the Nuget WinRTTimeZones package for the latest or 
// get the code at https://github.com/onovotny/WinRTTimeZones
namespace TimeZoneUtils
{
    public static class Utilities
    {
        private static DYNAMIC_TIME_ZONE_INFORMATION _eastern;

        static Utilities()
        {
            _eastern = EnumerateSystemTimeZones().First(tz => tz.TimeZoneKeyName == "Eastern Standard Time");
        }


        public static DateTimeOffset ConvertDateToEastern(DateTimeOffset date)
        {
            var currentTime = new SYSTEMTIME(date.UtcDateTime);


            TIME_ZONE_INFORMATION tzi;
            if (SafeNativeMethods.GetTimeZoneInformationForYear(currentTime.Year, ref _eastern, out tzi))
            {
                SYSTEMTIME local;
                if (SafeNativeMethods.SystemTimeToTzSpecificLocalTime(ref tzi, ref currentTime, out local))
                {
                    var dt = new DateTime(local.Year, local.Month, local.Day, local.Hour, local.Minute, local.Second, local.Milliseconds,
                                          DateTimeKind.Unspecified);

                    // calc offset = local hr/min - current hr/min
                    var hrs = local.Hour - currentTime.Hour;
                    var min = local.Minute - currentTime.Minute;

                    var dto = new DateTimeOffset(dt, new TimeSpan(hrs, min, 0));
                    return dto;
                }
            }

            var error = Marshal.GetLastWin32Error();
            Marshal.ThrowExceptionForHR(error);

            return date;
        }

        internal static IEnumerable<DYNAMIC_TIME_ZONE_INFORMATION> EnumerateSystemTimeZones()
        {
            var i = 0;
            while (true)
            {
                DYNAMIC_TIME_ZONE_INFORMATION tz;
                var result = SafeNativeMethods.EnumDynamicTimeZoneInformation(i++, out tz);

                if (result != 0)
                    yield break;

                yield return tz;
            }
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        internal struct DYNAMIC_TIME_ZONE_INFORMATION
        {
            [MarshalAs(UnmanagedType.I4)]
            public Int32 Bias;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
            public string StandardName;

            public SYSTEMTIME StandardDate;

            [MarshalAs(UnmanagedType.I4)]
            public Int32 StandardBias;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
            public string DaylightName;

            public SYSTEMTIME DaylightDate;

            [MarshalAs(UnmanagedType.I4)]
            public Int32 DaylightBias;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
            public string TimeZoneKeyName;

            [MarshalAs(UnmanagedType.I1)]
            public bool DynamicDaylightTimeDisabled;
        }

        [StructLayout(LayoutKind.Sequential)]
        internal struct SYSTEMTIME
        {
            [MarshalAs(UnmanagedType.U2)]
            public short Year;

            [MarshalAs(UnmanagedType.U2)]
            public short Month;

            [MarshalAs(UnmanagedType.U2)]
            public short DayOfWeek;

            [MarshalAs(UnmanagedType.U2)]
            public short Day;

            [MarshalAs(UnmanagedType.U2)]
            public short Hour;

            [MarshalAs(UnmanagedType.U2)]
            public short Minute;

            [MarshalAs(UnmanagedType.U2)]
            public short Second;

            [MarshalAs(UnmanagedType.U2)]
            public short Milliseconds;

            public SYSTEMTIME(DateTime dt)
            {
                dt = dt.ToUniversalTime(); // SetSystemTime expects the SYSTEMTIME in UTC
                Year = (short)dt.Year;
                Month = (short)dt.Month;
                DayOfWeek = (short)dt.DayOfWeek;
                Day = (short)dt.Day;
                Hour = (short)dt.Hour;
                Minute = (short)dt.Minute;
                Second = (short)dt.Second;
                Milliseconds = (short)dt.Millisecond;
            }
        }

        private static class SafeNativeMethods
        {
            [DllImport("kernel32.dll")]
            internal static extern bool SystemTimeToTzSpecificLocalTime([In] ref TIME_ZONE_INFORMATION lpTimeZoneInformation,
                                                                        [In] ref SYSTEMTIME lpUniversalTime,
                                                                        out SYSTEMTIME lpLocalTime);

            [DllImport("Advapi32.dll")]
            internal static extern int EnumDynamicTimeZoneInformation([In] int dwIndex, out DYNAMIC_TIME_ZONE_INFORMATION lpTimeZoneInformation);


            [DllImport("kernel32.dll")]
            internal static extern bool GetTimeZoneInformationForYear([In] short wYear,
                                                                      [In] ref DYNAMIC_TIME_ZONE_INFORMATION pdtzi,
                                                                      out TIME_ZONE_INFORMATION ptzi);
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        internal struct TIME_ZONE_INFORMATION
        {
            [MarshalAs(UnmanagedType.I4)]
            public Int32 Bias;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
            public string StandardName;

            public SYSTEMTIME StandardDate;

            [MarshalAs(UnmanagedType.I4)]
            public Int32 StandardBias;

            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
            public string DaylightName;

            public SYSTEMTIME DaylightDate;

            [MarshalAs(UnmanagedType.I4)]
            public Int32 DaylightBias;
        }
    }
}