Adding/subtracting time values of two Textboxes

you probably want to use TimeSpan because you don't need all the other stuff DateTime brings with it.

change this, and anything similar:
dtpEndTime.Text = (DateTime.Parse(dtpStartTime.Text).Add(DateTime.Parse("00:30:00"))).ToString();

to this:

dtpEndTime.Text = (TimeSpan.Parse(dtpStartTime.Text).AddMinutes(30)).ToString();

should work out.  if it dosn't could you also post the actual problem you are having with the way you are doing it now.

John

Thanks for the quick reply johnduff but, if I do use TimeSpan, then I don't have the addMinutes, addHours etc. functions. These only come with DateTime, I was using DateTime to keep my options open.
I am using the AddMinutes function to add and subtract the minutes, so that bit has been simplified.

I've been working on it since, it turns out that the effect I wanted was not occuring because I had it in the _SelectionChangeCommitted event rather than TextChanged. This is it now:

private void dtpHoursScheduled_TextChanged(object sender, EventArgs e)
        {
            dtpEndTime.Text = (DateTime.Parse(dtpStartTime.Text).Add(TimeSpan.Parse(dtpHoursScheduled.Text))).ToString();
            dtpEndTime.FormatString = "t";
        }

private void dtpStartTime_TextChanged(object sender, EventArgs e)
        {
            try
            {
                if (DateTime.Parse(dtpStartTime.Text).CompareTo(DateTime.Parse(dtpEndTime.Text)) >= 0)
                    dtpEndTime.Text = (DateTime.Parse(dtpStartTime.Text).AddMinutes(30)).ToString();
                dtpHoursScheduled.Text = (DateTime.Parse(dtpEndTime.Text).Subtract(DateTime.Parse(dtpStartTime.Text))).ToString();
                dtpEndTime.FormatString = "t";
            }
            catch (Exception ex)
            {
                //MessageBox.Show("Exception: " + ex.Message);
            }
        }

private void dtpEndTime_TextChanged(object sender, EventArgs e)
        {
            if (DateTime.Parse(dtpStartTime.Text).CompareTo(DateTime.Parse(dtpEndTime.Text)) >= 0)
                dtpStartTime.Text = (DateTime.Parse(dtpEndTime.Text).AddMinutes(-30)).ToString();
            dtpHoursScheduled.Text = (DateTime.Parse(dtpEndTime.Text).Subtract(DateTime.Parse(dtpStartTime.Text))).ToString();
            dtpStartTime.FormatString = "t";
        }


So I got it mostly working. The hours are now updating. But there are two problemss that are still occurring:
1) An exception is thrown when I go to select one of my items from the list. The program claims that the time "is not a valid DateTime format". But if I do nothing in the exception handler, I am still able to go ahead, select a value and it writes to the database successfully. What causes the exception?
2) When any of the fields - start, end, total - are modified by one of the above methods, their formatting changes from "09:00" to a full date + time string ie "25/04/2006 09:00:00" - how do I stop this changing?

I got it working. The toShortTimeString() came in very useful, so thanks johnduff for guiding me in the right direction.
The code still seems quite bloated though:

private void dtpHoursScheduled_TextChanged(object sender, EventArgs e)
        {
            try
            {
                dtpEndTime.Text = (DateTime.Parse(dtpStartTime.Text).Add(TimeSpan.Parse(dtpHoursScheduled.Text))).ToString();
                dtpEndTime.Text = DateTime.Parse(dtpEndTime.Text).ToShortTimeString();
                dtpStartTime.Text = DateTime.Parse(dtpStartTime.Text).ToShortTimeString();
            }
            catch (System.Exception ex)
            {
                string errMsg = ex.Message;
            }
        }

        private void dtpStartTime_TextChanged(object sender, EventArgs e)
        {
            string convertString = "";
            try
            {
                if (DateTime.Parse(dtpStartTime.Text).CompareTo(DateTime.Parse(dtpEndTime.Text)) >= 0)
                    dtpEndTime.Text = DateTime.Parse(dtpStartTime.Text).AddMinutes(30).ToShortTimeString();
                convertString = DateTime.Parse(dtpEndTime.Text).Subtract(DateTime.Parse(dtpStartTime.Text)).ToString();
                convertString = DateTime.Parse(convertString).ToShortTimeString();
                dtpHoursScheduled.Text = convertString;
            }
            catch (System.Exception ex)
            {
                string errMsg = ex.Message;
            }
        }

        private void dtpEndTime_TextChanged(object sender, EventArgs e)
        {
            string convertString = "";
            try
            {
                if (DateTime.Parse(dtpStartTime.Text).CompareTo(DateTime.Parse(dtpEndTime.Text)) >= 0)
                    dtpStartTime.Text = DateTime.Parse(dtpEndTime.Text).AddMinutes(-30).ToShortTimeString();
                convertString = DateTime.Parse(dtpEndTime.Text).Subtract(DateTime.Parse(dtpStartTime.Text)).ToString();
                convertString = DateTime.Parse(convertString).ToShortTimeString();
                this.dtpHoursScheduled.Text = convertString;
            }
            catch (System.Exception ex)
            {
                string errMsg = ex.Message;
            }
        }



There is a reason for me using the following snippet:
convertString = DateTime.Parse(dtpEndTime.Text).Subtract(DateTime.Parse(dtpStartTime.Text)).ToString();
convertString = DateTime.Parse(convertString).ToShortTimeString();
dtpHoursScheduled.Text = convertString;

If I don't use the variable converString and just do the operation on dtpHoursScheduled.Text itself, if I scroll the mousewheel up and down repeatedly in the program it causes a StackOverFlow exception. This way seems to resolve it I think, although I'm still not convinced.
Is the above section of code the only method for this? If no-one offers any suggestions then johnduff's hints will receive the points.