turn lights on and off using real time clock ds1307

``I am wondering if converting the time from 24hr to 12hr is going to interfere with the part of code that will turn on the ledpin and off at set times

#include <dht11.h> //this is for the temp and humidity sensor
#include <LiquidCrystal.h>
#include <Wire.h>
#include "RTClib.h"
#include <DateTime.h>
//#include <RTClib.h>

LiquidCrystal lcd(7, 8, 9, 10, 11, 12);
int hiTemp  = 30;//this  will activate relay 1 witch will turn on fan 1
int hiHumidity = 32;//this  will activate relay 2 witch will turn on fan 2
int lowTemp = 34;//this  will activate relay 3 witch will turn on heat lamp
int lowHumidity = 36;//this  will activate relay 4 witch will turn on mister
int ledPin = 13;

//PHOTOCELL VALUES
int sensorMin = 1023; // minimum photo sensor value 
int sensorMax = 0; // maximum photo sensor value
const int sensorPin = A0;// select the input pin for the photocell 
int sensorValue = 0;// variable to store the value coming from the photocell sensor
int tempPin = 2;//Temp and humidty pin
int ack;
int chk;
int count = 0;
char cmd = 0;
byte temppin = 0;

[[color=red]color=red][font=Verdana]byte startHour = 13;    // Set our start and end times
byte startMinute = 33;  // Don't add leading zero to hour or minute - 7, not 07
byte endHour = 13;      // Use 24h format for the hour, without leading zero
byte endMinute = 34;

byte validStart = 0;    // Declare and set to 0 our start flag
byte poweredOn = 0;     // Declare and set to 0 our current power flag
byte validEnd = 0;      // Declare and set to 0 our end flag
[/color]
dht11 DHT11;  //this is for the temp and humidity sensor

RTC_DS1307 RTC;


void setup()
{
  
    Serial.begin(9600);
    Wire.begin();
    RTC.begin();
    lcd.begin(20, 4);
    DHT11.attach(2);
   
    if (! RTC.isrunning())
    {
      lcd.setCursor(0,0);
   lcd.println("RTC is NOT running!");
    // following line sets the RTC to the date & time this sketch was compiled
     RTC.adjust(DateTime(__DATE__, __TIME__));   
    delay(4000);
   lcd.clear();
     
    }
     //PIN MODES   
   pinMode(hiTemp, OUTPUT);  //this  will activate relay 1 witch will turn on fan 1
   pinMode(hiHumidity, OUTPUT);//this  will activate relay 2 witch will turn on fan 2
   pinMode(lowTemp, OUTPUT);//this  will activate relay 3 witch will turn on heat lamp
   pinMode(lowHumidity, OUTPUT); //this  will activate relay 4 witch will turn on mister
 [color=red]  pinMode(ledPin, OUTPUT);// Set our LED as an output pin
    digitalWrite(ledPin, LOW); // Set the LED to LOW (off)[/color]
   //WELCOME SCREEN
   lcd.setCursor(0,0);
   lcd.print("Starting up");
   lcd.setCursor(0,1);
   lcd.print("*** WELCOME! ***");
   lcd.setCursor(0,3);
   lcd.print("Initializing");
     delay(4000);
   lcd.clear();
   

   
}
void loop()
{

  
//LIGHTS ON/OFF LOGIC
sensorValue = analogRead(sensorPin);    // read the analog input pin
  

  if (sensorValue < 300)
  {
   // Serial.println("LIGHTS OFF  ");
  //  Serial.println(sensorValue);    // print to screen
    lcd.setCursor(0,3);
    lcd.print ("Lights     OFF ");
    lcd.print (sensorValue);    
      delay(50);
 }
    else
{
    
      // Serial.println ("Lights  ON");
       // Serial.println(sensorValue);    // print to screen
   lcd.setCursor(0,3); 
   lcd.print ("Lights     ON  ");
   lcd.print (sensorValue);
      
}//END SETUP
  
  
 

//START DISPLAY OF TEMPERATURE READINGS   

{
//void checkTEMP();
  int chk = DHT11.read();  //this is for the temp and humidity sensor


 //  switch (chk)
  
   lcd.setCursor(0,2);
   lcd.print("Humidity    %  ");
   lcd.print((float)DHT11.humidity);
   
   delay(500);
   
   lcd.setCursor(0,1);
   lcd.print("Temperature F  ");
   lcd.print(DHT11.fahrenheit());

       delay(500);
   
}


    DateTime now = RTC.now();
    
if (now.month()<10)
{
  lcd.setCursor(0,0);
  lcd.print(" ");
  lcd.setCursor(1,0);      
  lcd.print(now.month(), DEC);
  lcd.setCursor(2,0);
  lcd.print("/  ");

}
  else
{

lcd.setCursor(0,0);
lcd.print(now.month(), DEC);
lcd.setCursor(2,0);
lcd.print("/  ");
}

if (now.day()<10)
{
    lcd.setCursor(3,0);
  lcd.print(" ");
  lcd.setCursor(4,0);
    lcd.print(now.day(), DEC);
    lcd.setCursor(5,0);
    lcd.print("/    ");
        lcd.setCursor(6,0);
    lcd.print(now.year(), DEC);
}
else
 {
 
  lcd.setCursor(3,0);
    lcd.print(now.day(), DEC);
    lcd.setCursor(5,0);
    lcd.print("/    ");
        lcd.setCursor(6,0);
    lcd.print(now.year(), DEC);     
       
 
}  
       
 
[color=red]    int hr_24, hr_12;// 	liudr
hr_24=now.hour();

if (hr_24==0) hr_12=12;
else hr_12=hr_24%12;

 if (hr_12<10)[/color]
 {
lcd.setCursor(12,0);
lcd.print("  ");
 lcd.setCursor(13,0);
lcd.print(hr_12, DEC);
lcd.setCursor(18,0);
if (hr_24<12) lcd.print("AM");
else lcd.print("PM");
 }
   else
 {
 
     lcd.setCursor(12,0);
    lcd.print("  ");
 lcd.setCursor(12,0); 
lcd.print(hr_12, DEC);

//... everything in between now last line:
lcd.setCursor(18,0);
if (hr_24<12) lcd.print("AM");
else lcd.print("PM");      
       

} 

 //     lcd.setCursor(11,0);
 //   lcd.print("   ");
// lcd.setCursor(12,0);
//lcd.print(now.hour(), DEC);
    lcd.setCursor(14,0);
    lcd.print(":  ");

    if (now.minute()<10)
{

 lcd.setCursor(15,0); 
lcd.print(" ");

 lcd.setCursor(16,0);
lcd.print(now.minute(), DEC);
    }
  else
{
 lcd.setCursor(15,0);
lcd.print(now.minute(), DEC);

 // lcd.setCursor(15,0);
    //lcd.print(now.minute(), DEC);
   delay(3000);
}


 

  



[color=red]  if (now.second() == 0){ // Only process if we have ticked over to new minute
  
    if (poweredOn == 0){  // Check if lights currently powered on 
    
      checkStartTime();    // If they are not, check if it's time to turn them on
    } 
    else
    {
      checkEndTime();      // Otherwise, check if it's time to turn them off
    }
  
    if (validStart == 1)
    { // If our timer is flagged to start, turn the lights on
      turnLightsOn();
    }
    if (validEnd == 1)
    {   // If our time is flagged to end, turn the lights off
      turnLightsOff();
    }
  }
  
  
  
  delay(1000);  

  }
byte checkStartTime() 
  {
  DateTime now = RTC.now();  // Read in what our current datestamp is from RTC
  
  if (now.hour() == startHour && now.minute() == startMinute) 
  {
    validStart = 1;  // If our start hour and minute match the current time, set 'on' flags
    poweredOn = 1;
  }
 else
 {
    validStart = 0;  // Otherwise we don't need to power up the lights yet
  }
  
  return validStart; // Return the status for powering up
}

byte checkEndTime() 
{
  DateTime now = RTC.now();  // Read in what our current datestamp is from RTC
  
  if (now.hour() == endHour && now.minute() == endMinute) 
  {
    validEnd = 1;    // If our end hour and minute match the current time, set the 'off' flags
    poweredOn = 0;
  }
  else 
  {
    validEnd = 0;    // Otherwise we don't need to power off the lights yet
  }
  
  return validEnd;   // Return the status for powering off
} 

void turnLightsOn() 
{
  digitalWrite(ledPin, HIGH);  // Turn on the LED
}

void turnLightsOff() 
{
  digitalWrite(ledPin, LOW);   // Turn off the LED

}[/color]

Please edit your post and put code tags around your code.

As your startHour is 13 then switching the RTC to 12 hour mode will effect the on/off time in that the hour will never get to 13 and the lights wont turn on. If you have a lower value for startHour (say 7) then the lights will come on at 7 in the morning and 7 in the evening, maybe not what you wanted. Best to keep the RTC in 24 hour mode and if you want a 12 hour display with AM/PM indicator then convert the hours value on the fly to 12 hour format.