Are you looking to remove the date on the blog page or only single posts? Could you provide a link to your site?
This should do the trick on single posts:
.single .posted-on {
display: none;
}
Don’t edit the theme files directly, otherwise your changes will be overwritten every time the theme is updated to the latest version.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.