You can configure the post date/time in the theme options.
Go to Atahualpa Theme Options - Post/Page Info Items and scroll down to "Post Info Items -
List of available post info items" until you see "%date('F jS, Y')%".
There's then some examples of how you can display the date and time, for example and I quote:-
%date('F j, Y, g:i a')% displays: December 10, 2008, 5:16 pm
You then need to insert this into the relevant place in the relevant Bylines further down the options page. (You're not limited to putting it in the Byline though.)
Hope this helps.