Converting time to 12 or 24 hr format

As per my recent task in my project I had to convert time to 24 hours format. The time was in format of 12:30 am or 2:30 pm. Hence I thought of writing a function that will convert as per requested formats. Please find the function below. I hope this will help in saving a good amount of time in finding the logic to convert time format.

<script type="text/javascript">
 function timeFormat(format, str){

     var timeParts = str.split(":");
     var fstPart = timeParts[0];
     var sndPart = timeParts[1];
  var sndPartNum = sndPart.match(/\d+/)[0];
  if(sndPart.match(/[a-zA-Z]+/)){
   var sndPartStr = sndPart.match(/[a-zA-Z]+/)[0];
  }else{
   var sndPartStr = '';
  }

     if(format==12){
   if(sndPart.match(/[a-zA-Z]+/) 
    && sndPart.match(/[a-zA-Z]+/).length 
    && fstPart <= 12){

     return fstPart + ":" + sndPartNum + " " + sndPartStr;
    }
   else if(!sndPart.match(/[a-zA-Z]+/)){
    if(fstPart <= 12){ // AM part
     return fstPart + ":" + sndPartNum + " am";
    }else if(fstPart > 12 && fstPart < 24){ // PM Part
     return parseInt(fstPart) - 12 + ":" + sndPartNum + " pm";
    }else{
     alert('the input value is not in proper format.');
     return false;
    }
   }else{
    alert('the input value is not in proper format.');
    return false;
   }
     }else if(format == 24){
   if(fstPart < 24){
    var hours = parseInt(fstPart);

         if(sndPartStr.toLowerCase() == "pm"){
           hours += 12;
         }

         return hours + ":" + sndPartNum;
   }else{
    alert('the input value is not in proper format.');
    return false;
   }
     }
 }


 // Calling the method with different type of values
 console.log('input: 2:30 pm. Convert to 24 hours format');
 console.log(timeFormat(24, '2:30 pm'));

 console.log('input: 2:30 am. Convert to 24 hours format');
 console.log(timeFormat(24, '2:30 am'));

 console.log('input: 22:30. Convert to 12 hours format');
 console.log(timeFormat(12, '22:30'));

 console.log('input: 2:30. Convert to 12 hours format');
 console.log(timeFormat(12, '2:30'));

 console.log('input: 2:30 am. Convert to 12 hours format');
 console.log(timeFormat(12, '2:30 am'));

 console.log('input: 18:00. Convert to 12 hours format');
 console.log(timeFormat(12, '18:00 am'));
</script>