The Date Posted is the first form field in the Post Interface, and how you use it has consequences.

The Date Posted dropdown is an HTML5 widget with a date and time picker.

The date you choose will become a permanent part of your post as the post URL will, in part, be comprised of this date. Unless you set a separate Post Sort Date, the Post Date will also be used to order your blog posts on the main page.

The Posted Date will also be used to automatically generate the blog's date posted value in your RSS feeds (Really Simple Syndication feeds).

The majority of RSS aggregators that will read your feeds (for example, Google's Feedburner application) rely upon this posted date, and they will not read your RSS feed if the date is too far in the past. This date should also be kept accurate for the best search results. 

If you don't want your post to be potentially read by RSS aggregators, you could set the post date far in the past. 

If you're taking a day or two to compose your post before releasing it, you should make sure that the Post Date is changed to the current date when you finally re-edit your post and check the release checkbox. Otherwise, your post will be considered a day or two old and may not be picked up by various RSS Feeds.

To avoid this, you may schedule a post a couple of days in the future when creating a post. 

Scheduling a post in the future

If you choose a future date when creating a post, you will be asked if you want this post will be automatically released in the future. If you click on yes and click submit, the post will be automatically released on the chosen date without having to return to the editor to release the post. Additionally, your post will be emailed to your subscribers on the preferred future date. Selecting no will revert the date-time to the current date.