Java script to calculate total hours in 12 hour format.

When I served there was no ":" in military time.

You need to split the entered strings into the hours and minutes and then reduce those values to minutes since midnight. Now adjust the out minutes by 12 hours and  subtract the in time in minutes since midnight.

You can use a custom Calculations script like:

function HHMM2Num(cValue)
{
var aValue = cValue.split(":");
return (aValue[0] * 60) + Number(aValue[1]);
}

event.value = 0;
var cIn = this.getField("In").valueAsString;
var cOut = this.getField("Out").valueAsString;
if(cIn && cOut) {
event.value = (HHMM2Num(cOut) + 720) - HHMM2Num(cIn);
}

And a custom format script of:

function Num2HHMM(nMinutes)
{
return util.printf("%,101.0f:%,102.0f", Math.floor(nMinutes / 60), nMinutes % 60);
}

event.value = Num2HHMM(event.value);

These statements are all standard Acrobat JavaScript and MDN JavaScript.

If the time period being computed includes the time event when there is a time change caused by changing to or form standard and summer (DST) time then the calculation script will not be correct And one needs to add additional code to adjust for the time change.

Your logic might have an error. The time from Noon (12:00 pm) to 12:59 pm is an afternoon time so it is within reason that an out time of 12:15 could be entered and add 12 hours to that will make the ending time in the next day.