Format the current local system time using the preferences set in the regional and language options portion of Control Panel

GetTimeFormatEx.cs

namespace Vurdalakov
{
    using System;
    using System.Diagnostics;
    using System.Runtime.InteropServices;
    using System.Text;

    public static class DateTimeApi
    {
        public static String GetCurrentTimeString(Boolean shortTime)
        {
            var stringBuffer = new StringBuilder(LOCALE_NAME_MAX_LENGTH + 1);
            var result = GetTimeFormatEx(LOCALE_NAME_USER_DEFAULT, shortTime ? TIME_NOSECONDS : 0, IntPtr.Zero, null, stringBuffer, stringBuffer.Capacity);
            if (result != 0)
            {
                return stringBuffer.ToString();
            }

            Trace.WriteLine($"GetTimeFormatEx() failed with error {Marshal.GetLastWin32Error()}");
            
            return DateTime.Now.ToString(shortTime ? "t" : "T", System.Globalization.CultureInfo.InstalledUICulture);
        }

        private const String LOCALE_NAME_INVARIANT = "";
        private const String LOCALE_NAME_USER_DEFAULT = null;
        private const String LOCALE_NAME_SYSTEM_DEFAULT = "!x-sys-default-locale";
        
        private const Int32 LOCALE_NAME_MAX_LENGTH = 85;

        private const Int32 TIME_NOMINUTESORSECONDS = 1;
        private const Int32 TIME_NOSECONDS = 2;
        private const Int32 TIME_NOTIMEMARKER = 4;
        private const Int32 TIME_FORCE24HOURFORMAT = 8;

        [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        private static extern Int32 GetTimeFormatEx(String locale, UInt32 dwFlags, IntPtr sysTime, String lpFormat, StringBuilder lpDateStr, Int32 cchDate);
    }
}