This gist contains code snippets that let you view IdleTime managed by the Windows OS and initialize it with mouse input.

Progam.cs

using System;
using System.Runtime.InteropServices;
using System.Threading;

namespace WindowsIdleTime
{
    class Program
    {
        [DllImport("user32.dll", SetLastError = true)]
        static extern uint SendInput(uint nInputs, ref INPUT pInputs, int cbSize);

        [DllImport("user32.dll", SetLastError = false)]
        private static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);

        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool GetCursorPos(out POINT p);

        [StructLayout(LayoutKind.Sequential)]
        struct LASTINPUTINFO
        {
            public uint cbSize;
            public int dwTime;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct POINT
        {
            public int x;
            public int y;
        }

        [StructLayout(LayoutKind.Sequential)]
        struct INPUT
        {
            public SendInputEventType type;
            public MouseKeybdhardwareInputUnion mkhi;
        }
        [StructLayout(LayoutKind.Explicit)]
        struct MouseKeybdhardwareInputUnion
        {
            [FieldOffset(0)]
            public MouseInputData mi;

            [FieldOffset(0)]
            public KEYBDINPUT ki;

            [FieldOffset(0)]
            public HARDWAREINPUT hi;
        }

        [StructLayout(LayoutKind.Sequential)]
        struct KEYBDINPUT
        {
            public ushort wVk;
            public ushort wScan;
            public uint dwFlags;
            public uint time;
            public IntPtr dwExtraInfo;
        }

        [StructLayout(LayoutKind.Sequential)]
        struct HARDWAREINPUT
        {
            public int uMsg;
            public short wParamL;
            public short wParamH;
        }

        struct MouseInputData
        {
            public int dx;
            public int dy;
            public uint mouseData;
            public MouseEventFlags dwFlags;
            public uint time;
            public IntPtr dwExtraInfo;
        }

        [Flags]
        enum MouseEventFlags : uint
        {
            MOUSEEVENT_MOVE = 0x0001,
            MOUSEEVENT_LEFTDOWN = 0x0002,
            MOUSEEVENT_LEFTUP = 0x0004,
            MOUSEEVENT_RIGHTDOWN = 0x0008,
            MOUSEEVENT_RIGHTUP = 0x0010,
            MOUSEEVENT_MIDDLEDOWN = 0x0020,
            MOUSEEVENT_MIDDLEUP = 0x0040,
            MOUSEEVENT_XDOWN = 0x0080,
            MOUSEEVENT_XUP = 0x0100,
            MOUSEEVENT_WHEEL = 0x0800,
            MOUSEEVENT_VIRTUALDESK = 0x4000,
            MOUSEEVENT_ABSOLUTE = 0x8000
        }
        enum SendInputEventType : int
        {
            InputMouse,
            InputKeyboard,
            InputHardware
        }


        private static DateTime LastInput
        {
            get
            {
                DateTime boolTime = DateTime.UtcNow.AddMilliseconds(-Environment.TickCount);
                DateTime lastInput = boolTime.AddMilliseconds(LastInputTicks);
                return lastInput;
            }
        }

        private static TimeSpan IdleTime
        {
            get
            {
                return DateTime.UtcNow.Subtract(LastInput);
            }
        }

        private static int LastInputTicks {
            get
            {
                LASTINPUTINFO lii = new LASTINPUTINFO();
                lii.cbSize = (uint)Marshal.SizeOf(typeof(LASTINPUTINFO));
                GetLastInputInfo(ref lii);
                return lii.dwTime;
            }
        }

        static void Main(string[] args)
        {
            Random r = new Random();
            while (true)
            {
                MouseMove(0, 0);
                Console.WriteLine(Math.Round(IdleTime.TotalSeconds));
                Thread.Sleep(r.Next(100, 10000));                       
            }
        }

        private static void ClickLeftMouseButton()
        {
            INPUT mouseDownInput = new INPUT();
            mouseDownInput.type = SendInputEventType.InputMouse;
            mouseDownInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENT_LEFTDOWN;
            SendInput(1, ref mouseDownInput, Marshal.SizeOf(new INPUT()));

            INPUT mouseUpInput = new INPUT();
            mouseUpInput.type = SendInputEventType.InputMouse;
            mouseUpInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENT_LEFTUP;
            SendInput(1, ref mouseUpInput, Marshal.SizeOf(new INPUT()));
        }
        private static void ClickRightMouseButton()
        {
            INPUT mouseDownInput = new INPUT();
            mouseDownInput.type = SendInputEventType.InputMouse;
            mouseDownInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENT_RIGHTDOWN;
            SendInput(1, ref mouseDownInput, Marshal.SizeOf(new INPUT()));

            INPUT mouseUpInput = new INPUT();
            mouseUpInput.type = SendInputEventType.InputMouse;
            mouseUpInput.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENT_RIGHTUP;
            SendInput(1, ref mouseUpInput, Marshal.SizeOf(new INPUT()));
        }

        private static void MouseMove(int dx, int dy)
        {
            INPUT mouseMove = new INPUT();
            mouseMove.type = SendInputEventType.InputMouse;
            mouseMove.mkhi.mi.dwFlags = MouseEventFlags.MOUSEEVENT_MOVE;
            mouseMove.mkhi.mi.dx = dx;
            mouseMove.mkhi.mi.dy = dy;
            SendInput(1, ref mouseMove, Marshal.SizeOf(new INPUT()));
        }
    }
}