Getting sum of salary when using date range filter

First of all, I have this test case http://live.datatables.net/veqetisa/1/edit
and I don't need to display the table on load. I need to display the table when the user choose dates. I did this by using hide and show functions and it is working perfectly fine, but is this a good way for this? Or there is a better way? Second, I don't need to display the table unless BOTH dates are entered not one date only, also I did this that as shown above in the test case, Is there a better way of doing this? Third, the important thing, when the user enters both dates, I need to get the sum of salary and display it rather than showing the original table (which is the date and salary). How can I do this?