Calculating difference in Time using Javascript, etc.

Question

I have an employee timesheet application where a clerk occasionally has to manually add a line. In the DB table, there are START_DATETIME and STOP_DATETIME fields (TIMESTAMP datatype) and the difference between these is the number of hours the employee gets paid for (obviously).
Right now, I have two calendar controls on a form (thanks for the updates in APEX 3.0 that improve TIMESTAMP handling BTW) to allow them to input these.
What the clerk wants to be able to do is select a START_DATETIME using the calendar control (easy) and then have the STOP_DATETIME calculated based on a third field - MINUTES. Anyone got any good ideas on how to do this, since (I think) it will have to be done client-side in JavaScript?

Answer 1

OK, here's what I ended up with:
function calc_time()
var oneMinute = 60 * 1000; // milliseconds in a minute
var oneHour = oneMinute * 60;
var oneDay = oneHour * 24;
// Get the IN_TIME value
var fld_in_time = document.getElementById("P27_START_DATETIME");
var var_in_time = new Date(fld_in_time.value);
// Get the Minutes, Calculate a new time
var fld_mins = document.getElementById("P27_MINS");
var var_new_time = var_in_time.getTime() + fld_mins.value * oneMinute;
var var_out_time = new Date(var_new_time);
// Debugging
//var fld_test_time = document.getElementById("P27_NEW_TIME");
//fld_test_time.value = var_out_time;
// Concatenating the OUT_TIME value
var fld_out_time = document.getElementById("P27_STOP_DATETIME");
var v_mm = var_out_time.getMonth();
if(v_mm < 10){
v_mm = "0" + v_mm
var v_dd = var_out_time.getDate();
if(v_dd < 10){
v_dd = "0" + v_dd
var v_yyyy = var_out_time.getFullYear();
var v_date = v_mm + '/' + v_dd + '/' + v_yyyy;
var v_hh24 = var_out_time.getHours();
if(v_hh24 < 10){
v_hh24 = "0" + v_hh24
var v_mm = var_out_time.getMinutes();
if(v_mm < 10){
v_mm = "0" + v_mm
var v_time = v_hh24 + ':' + v_mm;
fld_out_time.value = v_date + ' ' + v_time;
}