Refactor number one

Very early refactoring

if(document.getElementById('currentyear').value <= 2013) {

  if(document.getElementById('currentmonth').value == 12) {
    document.getElementById('currentmonth').value=13;
  }
  else if(document.getElementById('currentmonth').value == 11) {
    document.getElementById('currentmonth').value=12;
  }
  else if(document.getElementById('currentmonth').value == 10) {
    document.getElementById('currentmonth').value=11;
  }
  else if(document.getElementById('currentmonth').value == 9) {
    document.getElementById('currentmonth').value=10;
  }
  else if(document.getElementById('currentmonth').value == 8) {
    document.getElementById('currentmonth').value=9;
  }
  else if(document.getElementById('currentmonth').value == 7) {
    document.getElementById('currentmonth').value=8;
  }
  else if(document.getElementById('currentmonth').value == 6) {
    document.getElementById('currentmonth').value=7;
  }
  else if(document.getElementById('currentmonth').value == 5) {
    document.getElementById('currentmonth').value=6;
  }
  else if(document.getElementById('currentmonth').value == 4) {
    document.getElementById('currentmonth').value=5;
  }
  else if(document.getElementById('currentmonth').value == 3) {
    document.getElementById('currentmonth').value=4;
  }
  else if(document.getElementById('currentmonth').value == 2) {
    document.getElementById('currentmonth').value=3;
  }
  else if(document.getElementById('currentmonth').value == 1) {
    document.getElementById('currentmonth').value=2;
  }
  else {
    alert("CO2NS Error Code: MXNAZ-329701. Please inform site Administrator");
  }
  
  
  
  if((document.getElementById('currentyear').value == 2009) && (document.getElementById('currentmonth').value == 13)) {
    document.getElementById('currentmonth').value=1;document.getElementById('currentyear').value=2010;
  }
  if((document.getElementById('currentyear').value == 2010) && (document.getElementById('currentmonth').value == 13)) {
    document.getElementById('currentmonth').value=1;document.getElementById('currentyear').value=2011;
  }
  if((document.getElementById('currentyear').value == 2011) && (document.getElementById('currentmonth').value == 13)) {
    document.getElementById('currentmonth').value=1;document.getElementById('currentyear').value=2012;
  }
  if((document.getElementById('currentyear').value == 2012) && (document.getElementById('currentmonth').value == 13)) {
    document.getElementById('currentmonth').value=1;document.getElementById('currentyear').value=2013;
  }
  if((document.getElementById('currentyear').value == 2013) && (document.getElementById('currentmonth').value == 13)) {
    document.getElementById('currentmonth').value=1;document.getElementById('currentyear').value=2009;
    alert("CO2NS Site only performs CO2 Calculations up to, and including, 2013. Thank You!");
  }



  var yearvar = document.getElementById('currentyear').value;var monthvar = document.getElementById('currentmonth').value; $j.get('/detailed_data?showyear=' + yearvar + '&showmonth=' + monthvar + '&showsiteid=' + #{params[:site_id]}, function(data){});



}
else {
    alert("CO2NS Site only performs CO2 Calculations up to, and including, 2013. Thank You!");
}