Removing post date in the 2011 theme

First you will to create a child theme for your changes.

As esmi recommended, first create a child theme. This way you will not lose your customizations when you next update your theme/WP.

Then, try adding this to style.css of your child theme:
.by-author { display: none; }

.entry-date { display: none; }

Thank you very much.

you are welcome ??