If the user is not setting a publish from date initially, the page will be published. If the user goes back and makes a change or simply sets a publish from date, then from the date they entered forward the new content will be published, like a new version of it. For instance, I created a page at 7am yesterday without a publish from date in it. I realized at 9am that the page was showing, so I updated the page and set a publish from = tomorrow at 9am and saved. The version I created at 7am yesterday will show until the new version I have set to publish at 9am tomorrow.
The key is to ALWAYS set the publish from date prior to saving or publishing initially. If you don't you will have to Archive the document, set the publish from date, then save/publish again.