Publication Date is not decoration. It is the field the publications feed sorts on, so the date you put there decides where your work sits among everyone else's. Recent, Oldest and the feed's own default order all read this field, and the lists of publications on a member profile read it too.
Where the field is and what it defaults to
Open your publication and select Edit. The field is labelled Publication Date and it holds a date with no time, so publications sharing a day are not ordered against each other by hour. A new publication starts with the day you created it already filled in, which is why most publications sit in the feed at roughly the moment they went up.
A future date pins you to the top, and nothing refuses it
Nothing checks that the date has already happened. Set one in the future and the publication sits above every genuine publication in Recent until that day arrives, then drops back into place. It is accepted, it is not a trick worth using, and it makes the date on your own work wrong for as long as it lasts.
A backdated publication lands where that date lands
Put the original year on a thesis or a portfolio and it goes down the list to that year, which is usually a long way from the front. That is the honest trade: an accurate record, less visibility. Use the real date when the record matters, and today's date when you are publishing something now.
Where the date is shown
The date appears on the publication's card in the feed and on profiles, written as when it was published. For a PDF it also appears in the viewer's properties panel, behind any date stored inside the PDF file itself.
The publication's own page shows a different date. It displays the day the publication was uploaded to UNI rather than the Publication Date you set, so if you have changed the field the two disagree. The feed and the profile lists follow your date; the detail page does not.
Changing it later is allowed
You can edit the date whenever you like and the publication moves in the feed accordingly. Nothing else moves with it: the web address stays the same, so links and bookmarks keep working.