Check date, if time is +24hrs allow vote in PHP?

Hi there, I'm setting up a vote system on website using a html and php.

I can explain it in English

The user inserts there name, and picks their vote. On submition it then saves the username to an sql database with "username" and "datesubmitted, (the time in h:i:s a format);" as the fields.

How then would I add a query to the script to check whether it has been 24+ since the previous vote hours? Basically it looks at the "datesubmitted" so the user can vote again and if less than 24hrs, kill the script.

Thanks!