#include <cstdlib>
#include <Windows.h>
#include <fstream>
#include <string>
#include <cmath>
#include <ctime>

using namespace std;

// Open the Options file for global use.
static fstream File("myfile.txt", ios::in);

//////////////////////////////////////////////////////////////////////////////////////
// Name: StrtoInt
// Does: Turns a string of digits into a mathematically manipulatable integer.
// Take: string
// Rtrn: int
//////////////////////////////////////////////////////////////////////////////////////
int StrtoInt(string NumString)
{
	double Num = 0;               // The integer form of our string.
	int StrLen = NumString.length();   // The srings length
	
	// While the string still contains characters...
	while(StrLen--)
	{
		char OneDigit = NumString[0];            // Pull out the front most character.
		NumString = NumString.substr(1, StrLen); // Then chop it off of the string.
		
		// Put that digit through a switch statment...
		switch(OneDigit)
		{
            // ... and for each possible digit, add it to our Num variable.
            // Making sure to multiple it by a power of ten, to ensure
            // that it stays in the slot it's supposed to.
			case '1':
				Num += 1*pow((double)10, (double)StrLen);
				break;
			case '2':
				Num += 2*pow((double)10, (double)StrLen);
				break;
			case '3':
				Num += 3*pow((double)10, (double)StrLen);
				break;
			case '4':
				Num += 4*pow((double)10, (double)StrLen);
				break;  
			case '5':
				Num += 5*pow((double)10, (double)StrLen);
				break;  
			case '6':
				Num += 6*pow((double)10, (double)StrLen);
				break;  
			case '7':
				Num += 7*pow((double)10, (double)StrLen);
				break;  
			case '8':
				Num += 8*pow((double)10, (double)StrLen);
				break;  
			case '9':
				Num += 9*pow((double)10, (double)StrLen);
				break;  
			case '0':
				Num += 0;  //0*pow(10, x+1)
				break;
			default:
				Num *= -10000; //If an error occurs, Num will be some obscure negative number.
				break;
				
		}
	}
	
	// Return an Integer form of our number.
	return (int)Num;
}

//////////////////////////////////////////////////////////////////////////////////////
// Name: GetDays
// Does: Gets the total number of day cycles from the options file.
// Take:
// Rtrn: int
//////////////////////////////////////////////////////////////////////////////////////
int GetDays()
{
	int TotalDays = 0;   // the total day cycles.
	string Finder;       // A file searching string.
	
	// Dig through the file until we hit the end of the file.
	// for each "Day" we come across on the way, incriment TotalDays.
	while(!File.eof())
	{
		File >> Finder;
		if(Finder == "Day") TotalDays++;
	}
	
	// Reset the File stream, and return the total number of day cycles found.
	File.clear();
	File.seekg(0, ios::beg);
	
	return TotalDays;
}

//////////////////////////////////////////////////////////////////////////////////////
// Name: CheckTimes
// Does: Compares the current time to all of the listed times in the options file,
//       if there is a match, it returns the picture location for that time,
//       if not, it returns "No Change". 
// Take: int, string
// Rtrn: string
//////////////////////////////////////////////////////////////////////////////////////
string CheckTimes(int WhatDay, string ClockTime)
{
	string Finder;       // A file searching string.
	string Picture;      // The picture file path.
	string PictureTime;  // The time to change to the Picture.
	
	
	// This will dig through the file untill it gets to the correct day cycle.
	// it basicly just skips through each instance of the word "Day" until
	// it has goten to the current day cycle.
	while(WhatDay--)
	{
		while(Finder != "Day")
		{
			File >> Finder;
		}
		Finder = "";
	}
	
	// Now, with the cursor at the correct day cycle, we start looking at times
    // within that day cycle.
    // It stops looking once it hits the end of the file, or runs into the next day cycle.	
	while(Finder != "Day" && !File.eof())
	{
		File >> Finder;
		
		// Are we at a picture path?
		if(Finder == "Picture:")
		{
            // set that picture path to the Picture variable.
			File >> Picture;
			File >> Finder;
		}
		
		// Are we at a time designation?
		if(Finder == "Time:")
		{
            // set that time to the PictureTime variable.
			File >> PictureTime;
			PictureTime += ":00"; // add a ":00" on the end so that the background doesn't
			                      // constantly set for an entire minute.
		}
		
		// If the current time is equal to the PictureTime, then reset the File
		// stream and return that pictures path.
		if(PictureTime == ClockTime)
		{
			File.clear();
			File.seekg(0, ios::beg);
			return Picture;
		}
	}
	
	// If no picture was found to match the current time, reset the File stream
	// and return "No Change".
	File.clear();
	File.seekg(0, ios::beg);
	
	return "No Change";
}


  /////////////////////////////////////////////////////////////
 //                     The Main Function                   //
/////////////////////////////////////////////////////////////
int WINAPI WinMain (HINSTANCE hThisInstance, HINSTANCE hPrevInstance, LPSTR lpszArgument, int nFunsterStil)
{
	while(1)
	{
        // Set the current time.
		time_t CurTime = time(0);
		
		// Turn the Current time into a string.
		string TimeStr = asctime(localtime(&CurTime));
		
		// Set the Date from a portion of the Time string.
		string Date = TimeStr.substr(8, 2);
		
		// Set the Time in Hour:Minute:Second form from the Time string.
		string ClockTime = TimeStr.substr(11, 8);
		
		// Modulus the Current date by the total number of Day cycles.
		int ModDate = (StrtoInt(Date) % GetDays()) +1;
		
		// Get the background image. If no change is needed, will be set to "No Change".
		string BackImage = CheckTimes(ModDate, ClockTime);
		
		// If the background image needs to be changed, change it.
		if(BackImage != "No Change")
			SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, (void*)BackImage.c_str(), SPIF_UPDATEINIFILE);
	}
}

Day 1
Picture: C:\Main\Desktop\misc-lonelysky.bmp
Time:    03:00

Picture: C:\Main\Desktop\girl_g11.bmp
Time:    11:00

Picture: C:\Main\Desktop\redcross.bmp
Time:    18:00

Picture: C:\Main\Desktop\misc-ladyofthelake.bmp
Time:    23:00