internal class Program
    {
// Copied from the internal code, you won't need this:
		private static CultureInfo GetCultureInfo(string lang)
		{
			Debug.Assert(lang != null);
			if (lang.Length == 0)
			{
				return CultureInfo.CurrentCulture;
			}
			else
			{
				try
				{
					return new CultureInfo(lang);
				}
				catch (System.ArgumentException)
				{
					//					throw new XslTransformException(SR.Xslt_InvalidLanguage, lang);
					throw;
				}
			}
		}

		public static string MSFormatDateTime(string dateTime, string format, string lang, bool isDate)
		{
			try
			{
				// XsdDateTime is private, so fake this out and use DateTime
#if true
				DateTime dt = DateTime.Parse(dateTime);
#else
				if (!XsdDateTime.TryParse(dateTime, XsdDateTimeFlags.AllXsd | XsdDateTimeFlags.XdrDateTime | XsdDateTimeFlags.XdrTimeNoTz, out XsdDateTime xdt))
				{
					return string.Empty;
				}

				DateTime dt = xdt.ToZulu();
#endif

				CultureInfo ci = string.IsNullOrEmpty(lang) ?
									CultureInfo.CurrentCulture :
									GetCultureInfo(lang);

				if (isDate)
				{
					DateOnly dateOnly = DateOnly.FromDateTime(dt);
					return dateOnly.ToString(!string.IsNullOrEmpty(format) ? format : null, ci);
				}
				else
				{
					TimeOnly timeOnly = TimeOnly.FromDateTime(dt);
					if (!string.IsNullOrEmpty(format))
					{
						return timeOnly.ToString(format, ci);
					}
					else
					{
						// Duplicate what the Win32 GetTimeFormat API returns
						// with an empty format - the long time string.
						return timeOnly.ToLongTimeString();
					}
				}
			}
			catch (Exception ex) when (ex is ArgumentException or FormatException)
			{
				return string.Empty;
			}
		}

        static void Main(string[] args)
        {
            string result;
            var dtStr = DateTime.Now.ToString();
            Console.WriteLine($"Result: {dtStr}");

            result = MSFormatDateTime( dtStr, string.Empty, CultureInfo.CurrentCulture.Name, true);
            Console.WriteLine($"Result: {result}");
            result = MSFormatDateTime( dtStr, string.Empty, CultureInfo.CurrentCulture.Name, false);
            Console.WriteLine($"Result: {result}");

            result = MSFormatDateTime(dtStr, "D", "", true);
            Console.WriteLine($"Result: {result}");
            result = MSFormatDateTime(dtStr, "T", "", false);
            Console.WriteLine($"Result: {result}");

            result = MSFormatDateTime(dtStr, "d", "", true);
            Console.WriteLine($"Result: {result}");
            result = MSFormatDateTime(dtStr, "t", "", false);
            Console.WriteLine($"Result: {result}");
        }
    }