Error message on R4

I've been using an R4 on NDS for ages. All of a sudden its coming up with SYSTEM DATE ERROR! PLEASE CHECK NDS SYTSTEM DATE as soon as I try to turn it on. How can I do this if it won't even load up.......in easy to understand language please as I'm a novice. My grandson is beside himself as he is now restricted to the odd game I have on original ( which load ok ) Many thanks.

Turn your NDS date back a little ways until it works.
Or update your firmware.