delete date on posts

Thanks so much! It worked.

1.I am trying to have all posts in full width, I have placed the following css, but it does not work,

.single-post #secondary {display: none;}
.single-post #primary {
width: 100% !important;
}

2. And also I would like to not have sidebars and footers for mobile devices, and I have put the following css, but it removed none of them, what am I doing wrong?

@media (max-width: 979px) {
#sidebar, #footer {display: none;}
}

thanks in advance for your help!