How long is a day

In discussions with theists/deists, I run into a statement that I am not sure what would be an apprpriate response. My first reaction is "you're nuts", but I don't think that will be persuave.

So, I am looking for responses to the statement when discussing the age of the universe (13+ billions years old),

"How long is a day". This is a response I get from some theists/deists who try to reconcile the age of the universe with the 6 days in some scriptures. I sometimes try "calling a dog a cat does not change dogs to cats, a dog is a dog.", but I am hoping someone will suggest something better.