Speculation with portals
It's no different than if you place P1, go have dinner, watch a movie, get a good night's rest, take a shower, eat breakfast, and then place P2. Going through the portal will not take you back to the previous day just because you placed them at different times. The portal itself would have to time travel before we could even begin to consider whether portals could be used for that purpose.
Think of it like this:
P1 exists in the present
P2 is in the future
your great-great-great grandson exists in the future
You are in the present
Now, you can't walk into P1 to travel forward in time to exit from P2 the same way as you can't talk or interact with your future great-great-great grandson.
This is an awesome formatting convention to make talking about this stuff a bit easier to read. I hope this idea is widely copied. 
I agree with espen and Shmitz. The portals' behavior is consistently matter-of-fact, even blas?, with regard to time. To put it succinctly:
Two linked portals exist iff (which means "if and only if") both the blue portal and the orange portal are deployed on nonmoving surfaces simultaneously.
When they exist, any object that passes through one will appear at the other in the same instant in time.
When only one portal is deployed, it is clouded, and cannot be entered.
The key word up there is simultaneously. If we're supposing that you're able to time travel, and deploy the portals at different times, that's great. As soon as they are both deployed at the same moment, they will open and be passable as an instant bridge through space. They will not open before, and they will not transport you through time.
P1 exists in the present
P2 is in the future
your great-great-great grandson exists in the future
You are in the present
Now, you can't walk into P1 to travel forward in time to exit from P2 the same way as you can't talk or interact with your future great-great-great grandson.
But if its like this:
P1 is in the past
P2 is in the present
That is different
No it's not. It's the exact same thing. We're just the great-great-great grandson.
No see understand that if you place the Blue Portal in the past and then the Orange Portal in the future, than once you step through the Orange portal you arrive in the blue portal, within the same time period, the blue portal just stays there, and ages. Theres no time travel at all. Instead the blue Portal will update with time itself, and you will arrive in the area that the blue portal sent you, in the current time.
In essence, folding 3d space between a pair of portals creates a cube between them (with the portals being oppisite points on the cubes) that "fold cubes" created by other portal pairs cannot cross. This still allows two portals inside the cube, and (usually) two portals outside the cube, but a portal in the cube and a portal outside the cube is a no-no. Anyone get what I mean?
I've got more to add to this theory. This only holds if the fabric of the universe is inflexible. However, if the universe is flexible, the ASHPD would use incredible amounts of power to squeeze two points together.
Also, if you had one O and two Os, I believe anything sent through the blue portal would come out one of the orange portals, but not both; Much like the double slit experiment in quantum mechanics.
Does that mean that an item would go through both when it's not being observed?
Exactly. Though the collisions between molecules in the object is enough to constitute an "observation".
P1 is in the past
P2 is in the present
That is different
Nope. It's the same, as Schmitz stated, we're just the great-great-great grandson. In our time, our great-great-great grandfather exists. He has aged through time and existed a long time before you came to be. You can talk to him (through his burial place), as you can walk from O to O.