How to handle a multitasking on ATMEGA128 ?

void task0(void)
{ 
	
	typedef enum {
		DISPLAY_TIME,
		SET_WEEK_DAY,
		SET_MONTH,
		GO_BACK
		
	} my_states;
	
    //static uint8_t state;
	static my_states state_var = DISPLAY_TIME;
    int button_input;
   	 switch (state_var) {

		 DISPLAY_TIME :
		 display_time();//display time
		             	 
		button_input = button_is_pressed();//read button
		  if (button_input==1)//if button pressed, state = b;
		  {
		    state_var = SET_WEEK_DAY;	  
		  }else  //else
		  {
			  display_time();//read ds1307 into time structure
			                 //read time struct and display on lcd
		  }
		 
		 break;        // break;

		 SET_WEEK_DAY:  //set date
		 lcd_cmd(0x01);//clear lcd
		 lcd_cmd(0x0F);//view cursor
		 lcd_xy(0,0);//put in the position 0,0 to edit
		 _delay_ms(250);
		 lcd_string("Enter new day:");
		 lcd_xy(1,0);
		 lcd_string(weekday[ds1307_addr[3]]);//display day of the week
		 lcd_xy(1,0);//return the cursor to the front
		 button_input = button_is_pressed();//read button
		 if (button_input==2) //if button 2 pressed 
		 {
			 lcd_string(weekday[ds1307_addr[3]+1]);//increase day of the week
		 }
		 state_var = SET_MONTH;
		 break;

		 SET_MONTH:
		 state_var =GO_BACK;
		 break;

		 GO_BACK:
		 state_var = DISPLAY_TIME;
		 break;

		 default:
		 break; //default
	 }//end of switch
task_timers[0] = 10;      //every 100ms

  reset_task(0);
}

int main(void)
{
  //   start at least one task here
    //
           set_task(0);   //task0 runs display the time
	   set_task(1);  //put tick on output

    //      main loop
	
    while(1)
	{
	     if (tick_flag)
	     {
		     tick_flag = 0;
		     task_dispatch();              // well....
	     }
	  
	}
}