Help with calculations script

Hi. I have a script for calculations that are giving me a problem.  I enter time 1 and the other 3 times are calculated.  I can also, edit times and the following times are calculated.  The problem I am having is that Time 4 calculation doesn't always register.  I have to delete and reenter the prior times to get it to calculate.  Can someone recommend a solution? Thanks  

//start of code for when end time entered manually

var start = this.getField("end").valueAsString;

if (start=="") event.value = "";
else {
	
var t1c = Number(start.substr(0,2)); 
var t2c = Number(start.substr(2,2));
var hrc = t1c;
var minc = t2c;


if(t2c > 30){
    hrc = t1c + 1;
    minc = "0" + ((t2c - (-30) - 60));
}
if(t2c == "30"){
    hrc = t1c + 1;
    minc = "00";
}
if(t2c < 30){
    hrc = t1c;
    minc = t2c + 30;		
}
if(t2c > 40){
    hrc = t1c + 1;
    minc = ((t2c - (-30) - 60));
}
if(t2c == "40"){
    hrc = t1c + 1;
    minc = ((t2c - (-30) - 60));
}

var hr2c = hrc;
var min2c = minc;

if(hr2c < 10){
  hr2c = "0" + hrc;
  min2c = minc;
}
if(hr2c > 10){
  hr2c = hrc;
  min2c = minc;
}


//start of code for when finish time entered manually

var start = this.getField("finish").valueAsString;

if (start=="") event.value = "";
else {

var t1b = Number(start.substr(0,2)); 
var t2b = Number(start.substr(2,2));
var hrb = t1b;
var minb = t2b;

if(t2b > 29){
    hrb = t1b + 1;
    minb = "0" + ((t2b - (-31) - 60));
}
if(t2b == "29"){
    hrb = t1b + 1;  
    minb = "00";
}
if(t2b < 29){
    hrb = t1b;
    minb = t2b + 31;
}
if(t2b > 39){
    hrb = t1b + 1;
    minb = ((t2b - (-31) - 60));
}
if(t2b == "39"){
    hrb = t1b + 1;
    minb = ((t2b - (-31) - 60));
}

  var hr2b = hrb;
  var min2b = minb;

if(hr2b < 10){
  hr2b = "0" + hrb;
  min2b = minb;
}
if(hr2b > 10){
  hr2b = hrb;
  min2b = minb;
}




//start of code for when procend stop time entered manually

var start = this.getField("procend").valueAsString;

if (start=="") event.value = "";
else {
	
var t1 = Number(start.substr(0,2)); 
var t2 = Number(start.substr(2,2));
var hr = t1;
var min = t2;


if(t2 > 28){
    hr = t1 + 1;
    min = "0" + ((t2 - (-32) - 60));
}
if(t2 == "28"){
    hr = t1 + 1;  
    min = "00";
}
if(t2 < 28){
    hr = t1;
    min = t2 + 32;
}
if(t2 > 38){
    hr = t1 + 1;
    min = ((t2- (-32) - 60));
}
if(t2 == "38"){
    hr = t1 + 1;
    min = ((t2- (-32) - 60));
}

  var hr2 = hr;
  var min2 = min;

if(hr2 < 10){
  hr2 = "0" + hr;
  min2 = min;
}
if(hr2 > 10){
  hr2 = hr;
  min2 = min;
}

var final = hr2.toString() + min2.toString() ;

if (event.source === this.getField("procend")){
        event.value = final;
}        
}

//end of code for when procend time entered manually


var finalb = hr2b.toString() + min2b.toString() ;

if (event.source === this.getField("finish")){
        event.value = finalb;
}        
}

//end of code for when finish time entered manually


var final = hr2c.toString() + min2c.toString() ;

if (event.source === this.getField("end")){
        event.value = final;
}       
}

//end of code for when end time entered manually