"The specified date is invalid" will be perfectly well understood by essentially everyone who encounters it as a website error message.
You'd probably get good understanding with the unusual but brief and informative
If you wanted to be complete and/or pedantic you could say
where xxx is the user input data.
This carries a very slightly stronger suggestion that what is wrong is the formatting or genuine-date nature of the entry.
Without this the entry could be a properly formatted date entry and refer to a real-world day but still be "invalid". For example,
Departure date ? : 11/11/2013
Return date ? : 12/12/2012 -> The specified date is invalid
(Because return is before departure)
Date of birth ? : 11/11/1812 -> The specific date is invalid
(Because not even I am that old)
The above is not an absolute statement about date formatting or real-world date being invalid, but making this certain would require a much more complex and pedantic sentence, which is not justified or useful in the context of an error message.