Alf's solution is, naturally, excellent
A solution that i used for a similar problem involved using reg expressions to change td to a date type, storing to array, then sorting on that newly-defined array.
The reason why i adopted this approach was that i did not anticipate users wanting to order on this column often enough to justify having an extra span in the HTML.
Of course meant another chunky JS function
but when i had a table 250 rows long didnt matter
Both are good depends 1) how comfortable you are with reg exp and JS and 2) what trade-off you expect or desire (ie more HTML or more code)
Alex