Cannot import date correctly from excel by using TicketImportPlugin

[ITGirl]

I'm trying to import some tickets information to Traclightning from
excel.
But it seems that the date information(ticket's due_assign, due_close)
cannot be imported correctly, they are imported as text.
The other fields can be imported correctly.

I've installed the TicketImportPlugin and xlrd library like the
following link's description.

http://trac-hacks.org/wiki/TicketImportPlugin

Maybe this is a very simple question,
Could someone tell me how to resolve this? Please help me.
Thank you very much.