having trouble with - if one field empty then another field should be empty

Im having trouble with this code. What i am doing is taking a user entered time and adding 10 minutes to it and displaying the new value in a second field. However if the user entered field is empty then the second field should also be empty.  Here is the code i am using and it is located in the custom calculation field of the second field ( have also tried it in the validation part of the second field.)

var strStart = this.getField("Break2O").value;
strStart = strStart + " 1/1/70";


if(strStart.length)
{
var timeStart = util.scand("h:MM tt m/d/yy", strStart);
var oNext = timeStart
oNext.setMinutes(oNext.getMinutes()+ 10);
var oneHour = 60 * 60 * 1000;
var sNext = util.printd("h:MM tt", oNext);
event.value = sNext;
}
else {
//if (strStart = "");{
event.value = "0";
}
  //event.value = this.getField("Break2O").value;

any help is greatly appreciated. I have some more custom calculation stuff to do with the same project (Time Card Calculation) if anyone can help me out.