I have a .net 2.0 ascx control with a start time and end time textboxes. The data is as follows:
txtStart.Text = 09/19/2008 07:00:00
txtEnd.Text = 09/19/200
Once your textbox date formats are known in advance, you can use Matt Kruse's Date functions in Javascript to convert the two to a timestamp, subtract and then write to the resulting text box.
Equally the JQuery Date Input code for stringToDate could be adapted for your purposes - the below takes a string in the format "YYYY-MM-DD" and converts it to a date object. The timestamp (getTime()) of these objects could be used for your calculations.
stringToDate: function(string) {
var matches;
if (matches = string.match(/^(\d{4,4})-(\d{2,2})-(\d{2,2})$/)) {
return new Date(matches[1], matches[2] - 1, matches[3]);
} else {
return null;
};
}