I went to update my customers order and when I clicked on the update button I got a blank screen. So I hit the back button and it looked like my update went through. I went into my cpanel and saw this error.

[Tue Sep 27 18:56:13 2011] [error] [client 67.4.66.137] PHP Fatal error: Call to undefined function date_diff() in /home/fromprop/public_html/secretadmin/orders.php on line 152, referer: http://frompropertorocker.com/secret...13&action=edit

What does this mean and how do I fix it?

I never got this error before.

Thanks!