Make posts display differently based on date

Is there a way to make posts display differently according to date posted? I would like to edit my theme to make my main content wider than it is now, but I don’t want to mess up the display of any posts older than today. So, I’d like posts dated prior to today to continue displaying at 700px wide, and posts made after today to display at 1000px wide. I think I would need a when/else rule in my css but I’ve never done this and don’t want to mess anything up! My site is https://goldlionstyle.com/