using System;

using System.Globalization;

​

public class Program

{

    private static readonly CultureInfo usaCulture = CultureInfo.GetCultureInfo("en-US");

​

    public static void Main()

    {

        var utc = DateTime.UtcNow;

        var targetTimeZone = TimeZones.EST;

        var utcOffset = targetTimeZone.GetUtcOffset(utc);

        Console.WriteLine($"UTC offset for '{targetTimeZone}' is {utcOffset}");

        Console.WriteLine();

        Console.WriteLine("{0, -50} {1, 25:o}", TimeZones.UTC, utc);

        Console.WriteLine("{0, -50} {1, 25:o} (note no 'Z')", targetTimeZone, TimeZoneInfo.ConvertTimeFromUtc(utc, targetTimeZone));

        Console.WriteLine();

        Console.WriteLine("SOD {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfDay(targetTimeZone));

        Console.WriteLine("EOD {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfDay(targetTimeZone));

        Console.WriteLine("SOM {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfMonth(targetTimeZone));

        Console.WriteLine("EOM {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfMonth(targetTimeZone));  

        Console.WriteLine("SOW {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfWeek(targetTimeZone));

        Console.WriteLine("EOW {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfWeek(targetTimeZone));       

        Console.WriteLine("SOP-D {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfPeriod(Period.Day, targetTimeZone)); 

        Console.WriteLine("EOP-D {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfPeriod(Period.Day, targetTimeZone));   

        Console.WriteLine("SOP-W {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfPeriod(Period.Week, targetTimeZone));    

        Console.WriteLine("EOP-W {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfPeriod(Period.Week, targetTimeZone));

        Console.WriteLine("SOP-M {0, -50} {1, 25:o}", targetTimeZone, utc.ToStartOfPeriod(Period.Month, targetTimeZone));   

        Console.WriteLine("EOP-M {0, -50} {1, 25:o}", targetTimeZone, utc.ToEndOfPeriod(Period.Month, targetTimeZone)); 

    }

}

​

    public static class TimeZones

    {

        public static readonly TimeZoneInfo UTC = TimeZoneInfo.Utc;

        public static DateTime ToUTC(this DateTime dateTime) => dateTime.ToTimeZone(UTC);

​

        public static readonly TimeZoneInfo EST = GetTimeZoneCrossPlatform("Eastern Standard Time", "US/Eastern");

        public static DateTime ToEST(this DateTime dateTime) => dateTime.ToTimeZone(EST);

​

        public static readonly TimeZoneInfo CST = GetTimeZoneCrossPlatform("Central Standard Time", "US/Indiana-Starke");

        public static DateTime ToCST(this DateTime dateTime) => dateTime.ToTimeZone(CST);

​

        public static readonly TimeZoneInfo MDT = GetTimeZoneCrossPlatform("Mountain Standard Time", "US/Mountain");

        public static DateTime ToMDT(this DateTime dateTime) => dateTime.ToTimeZone(MDT);

​

        public static readonly TimeZoneInfo PST = GetTimeZoneCrossPlatform("Pacific Standard Time", "US/Pacific");

        public static DateTime ToPST(this DateTime dateTime) => dateTime.ToTimeZone(PST);

​

        public static TimeZoneInfo GetTimeZoneCrossPlatform(string windowsTimeZoneId, string unixTimeZoneId)

        {

            if (string.IsNullOrEmpty(windowsTimeZoneId))

            {

                throw new ArgumentException($"'{nameof(windowsTimeZoneId)}' cannot be null or empty.", nameof(windowsTimeZoneId));

            }

            if (string.IsNullOrEmpty(unixTimeZoneId))

            {

                throw new ArgumentException($"'{nameof(unixTimeZoneId)}' cannot be null or empty.", nameof(unixTimeZoneId));

            }

​

            try

            {

                return TimeZoneInfo.FindSystemTimeZoneById(windowsTimeZoneId);

            }

            catch (Exception)

            {

                // https://github.com/dotnet/runtime/issues/20523

                return TimeZoneInfo.FindSystemTimeZoneById(unixTimeZoneId);

            }

        }

        public static DateTime NewDateTime(this TimeZoneInfo timeZone, int year, int month, int day)

        {

            var tzDateTime = new DateTime(year, month, day, 0, 0, 0, DateTimeKind.Unspecified);

​

            var utcResult = new DateTimeOffset(tzDateTime, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime;

​

            return utcResult;

        }

​

        public static DateTime NewDateTime(this TimeZoneInfo timeZone, int year, int month, int day, int hour, int minute, int second)

        {

            var tzDateTime = new DateTime(year, month, day, hour, minute, second, DateTimeKind.Unspecified);

​

            var utcResult = new DateTimeOffset(tzDateTime, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime;

​

            return utcResult;

        }

​

        public static DateTime NewDateTime(this TimeZoneInfo timeZone, int year, int month, int day, int hour, int minute, int second, int millisecond)

        {

            var tzDateTime = new DateTime(year, month, day, hour, minute, second, millisecond, DateTimeKind.Unspecified);

​

            var utcResult = new DateTimeOffset(tzDateTime, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime;

​

            return utcResult;

        }   

    }

​

    public static class DateTimeExtensions

    {

        public static DateTime ToTimeZone(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            return TimeZoneInfo.ConvertTime(dateTime, timeZone);

        }

​

        public static DateTime ToStartOfDay(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzStartOfDay = tzDateTime.Date;

​

            var utcResult = new DateTimeOffset(tzStartOfDay, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime;

​

            return utcResult;

        }

​

        public static DateTime ToEndOfDay(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzEndOfDay = new DateTime(tzDateTime.Year, tzDateTime.Month, tzDateTime.Day, 23, 59, 59, 999, tzDateTime.Kind);

​

            var utcResult = new DateTimeOffset(tzEndOfDay, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime;

​

            return utcResult;

        }

​

        public static DateTime ToStartOfMonth(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzStartOfMonth = new DateTime(tzDateTime.Year, tzDateTime.Month, 1, tzDateTime.Hour, tzDateTime.Minute, tzDateTime.Second, tzDateTime.Millisecond, tzDateTime.Kind);

​

            var utcResult = new DateTimeOffset(tzStartOfMonth, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime.ToStartOfDay(timeZone);

​

            return utcResult;

        }

​

        public static DateTime ToEndOfMonth(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzEndOfMonth = new DateTime(tzDateTime.Year, tzDateTime.Month, DateTime.DaysInMonth(tzDateTime.Year, tzDateTime.Month), tzDateTime.Hour, tzDateTime.Minute, tzDateTime.Second, tzDateTime.Millisecond, tzDateTime.Kind);

​

            var utcResult = new DateTimeOffset(tzEndOfMonth, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime.ToEndOfDay(timeZone);

​

            return utcResult;

        }

​

        public static DateTime ToStartOfWeek(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzStartOfWeek = tzDateTime.AddDays(-(int)tzDateTime.DayOfWeek + (int)System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek);

​

            var utcResult = new DateTimeOffset(tzStartOfWeek, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime.ToStartOfDay(timeZone);

​

            return utcResult;

        }

​

        public static DateTime ToEndOfWeek(this DateTime dateTime, TimeZoneInfo timeZone)

        {

            var tzDateTime = dateTime.ToTimeZone(timeZone);

            var tzEndOfWeek = tzDateTime.AddDays(-(int)tzDateTime.DayOfWeek + (int)System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek).AddDays(6);

​

            var utcResult = new DateTimeOffset(tzEndOfWeek, timeZone.GetUtcOffset(tzDateTime)).UtcDateTime.ToEndOfDay(timeZone);

​

            return utcResult;

        }

​

        public static DateTime ToStartOfPeriod(this DateTime dateTime, Period period, TimeZoneInfo timeZone)

        {

            switch (period)

            {

                case Period.Day:

                    return dateTime.ToStartOfDay(timeZone);

                case Period.Week:

                    var shifted = dateTime.ToStartOfWeek(timeZone);

                    if (shifted.Month < dateTime.Month)

                    {

                        return dateTime.ToStartOfMonth(timeZone);

                    }

                    return shifted.ToStartOfDay(timeZone);

                case Period.Month:

                    return dateTime.ToStartOfMonth(timeZone);

                default:

                    throw new ArgumentOutOfRangeException(nameof(period), $"Unexpected Period '{period}'.");

            }

        }

​

        public static DateTime ToEndOfPeriod(this DateTime dateTime, Period period, TimeZoneInfo timeZone)

        {

            switch (period)

            {

                case Period.Day:

                    return dateTime.ToEndOfDay(timeZone);

                case Period.Week:

                    var shifted = dateTime.ToEndOfWeek(timeZone);

                    if (shifted.Month > dateTime.Month)

                    {

                        return dateTime.ToEndOfMonth(timeZone);

                    }

                    return shifted.ToEndOfDay(timeZone);

                case Period.Month:

                    return dateTime.ToEndOfMonth(timeZone);

                default:

                    throw new ArgumentOutOfRangeException(nameof(period), $"Unexpected Period '{period}'.");

            }

        }

​

        public static DateTime ToBeginningOfNextPeriod(this DateTime dateTime, Period period, TimeZoneInfo timeZone)

        {

            return dateTime.ToEndOfPeriod(period, timeZone).AddDays(1).ToStartOfDay(timeZone);

        }

    }

​

    public enum Period

    {

        Day,

        Week,

        Month

    }

​