using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        // Input values

        DateTimeOffset utc = DateTimeOffset.UtcNow;

        TimeZoneInfo zone = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");

        // Convert with full fidelity using only DateTimeOffset values

        DateTimeOffset converted = TimeZoneInfo.ConvertTime(utc, zone);

        // Get the start of day using the method from https://stackoverflow.com/a/49988688

        DateTimeOffset startOfDay = GetStartOfDay(converted.DateTime, zone);

        // Output the results

        Console.WriteLine("Original values:");

        Console.WriteLine("Chicago: {0:o}   UTC: {1:o}   Local: {2:o}", converted, utc, utc.ToLocalTime());

        Console.WriteLine();

        Console.WriteLine("At start of day in Chicago:");

        Console.WriteLine("Chicago: {0:o}   UTC: {2:o}   Local: {2:o}", startOfDay, startOfDay.ToUniversalTime(), startOfDay.ToLocalTime());

    }

​

    static DateTimeOffset GetStartOfDay(DateTime dt, TimeZoneInfo tz)

    {

        // Work in the time zone provided

        if (dt.Kind != DateTimeKind.Unspecified)

        {

            dt = TimeZoneInfo.ConvertTime(dt, tz);

        }

​

        // Start with assuming midnight

        var d = dt.Date;

        // Check for the time being invalid and handle if so

        if (tz.IsInvalidTime(d))

        {

            // the gap is *usually* 1hr, but not always, so calculate it

            var gap = tz.GetUtcOffset(dt.AddDays(1)) - tz.GetUtcOffset(dt.AddDays(-1));

            // advance forward by the amount of the gap

            d = d.Add(gap);

        }

​

        // Also check for the time being ambiguous, such as in a fall-back transition.

        // We want the *first* occurrence, which will have a *larger* offset

        var offset = tz.IsAmbiguousTime(d) ? tz.GetAmbiguousTimeOffsets(d).OrderByDescending(x => x).First() : tz.GetUtcOffset(d);

        // Now we know when the date starts precisely

        return new DateTimeOffset(d, offset);

    }

}