Additionally, the content of European patent
applications as filed, the dates of filing of which are prior to the
date referred to in paragraph 2 and which were published on or after
that date, shall be considered as comprised in the state of the art.
At the EPO, unpublished EP applications filed before the date of filing of an application are prior art for novelty but not prior art for inventive step. The theory is that even if it is unpublished, if someone filed on "the same thing" before you did your invention is not novel. However, to be in the state of the art for purposes of a document pointing someone in the direction of your invention (inventive step) it must actually have been available to the public before your filing.
This is not the same in the US. If a reference is prior art for section 102 purposes, it is prior art for section 103 purposes.
Regarding priority date vs actual filing date -
Article 89 Effect of priority right The right of priority shall
have the effect that the date of priority shall count as the date of
filing of the European patent application for the purposes of Article
54, paragraphs 2 and 3, and Article 60, paragraph 2
So a proper claim of priority moves the "effective date" back from the actual fining date to the priority date. One concern is whether or not the priory claim is to a filing that really qualifies under the Paris Agreement Article 4.
Also, In the EPO's Guidelines for Examination Part G , Chapter IV paragraph-3.
Date of filing or priority date as effective date It should be noted
that "date of filing" in Art. 54(2) and (3) is to be interpreted as
meaning the date of priority in appropriate cases (see F‑VI, 1.2)