It seems Tridion forgot about this validation check. However you could build it yourself by either:

* Create a function on Component and Page publish pre which checks whether the publish date is not in the past and throws an error through the GUI.
* Edit the CMS GUI and add the validation yourself (\tridion\web\views\asp\popups\publishtransactions.asp).

I would prefer the first option as it will keep working after an update of the GUI.

I have one question: In Tridion we can publish the contents (page or component) for the past date (By using Publish later option) and its allow the user to set the past date.

why this function is not throwing an error?

please i need a urgent reply for this query.