Wolverine:Origins/X1/X2/X3/The Wolverine all happen in one timeline.
Let's say that 'First Class' happens in a different timeline (hence the many differences in continuity between it and the other films).
'Days of Future Past' follows on from 'First Class', ten years later in the 1970's. In this film some mutants will appear from 'the future' (or so it seems). This future will actually be from the other timeline; two years after the events of 'The Wolverine' (and so ten years after 'X3').
So, this is not conventional timetravel but rather travel from 'The Original Trilogy' timeline, which from the perspective of 'First Class' is an alternative or parallel timeline.
This allows 'Days of Future Past' to be a direct sequel to 'First Class' and a spin-off pre/sequel to the original trilogy(!). Director Bryan Singer has said "it's not a sequel, it's not a prequel, it's a 'inbetwequel'."
Singer explains; "We've cracked it in a way that it makes sense. I had a two-hour conversation with James Cameron about time travel, string theory, multiverses and all that. You have to create your rules and stick wtih them. That's why Terminator and Back To The Future work so well. And there are certain mechanisms in X-Men, certain powers, perceptions and characters, that make this possible."
Singer; "I'm taking into account every movie. I'm not just grabbing my first two movies and 'First Class' and smashing them together. I'm taking into account the entire universe as it’s been laid out so far on the screen, and really respecting it and trying to work with that. People took things in various directions, so there’s some clean-up, but ultimately I’m not just ignoring them either."
Singer continues, "The movie takes place at different times. It deals with, I don't want to say time travel, but time displacement. A form of time travel which is steeped more in the X-Men [mythos]".
It is not yet known how this film will link the whole franchise together but my thoughts/hopes are that at the end of 'Days Of Future Past' we could have a kind of ultimate collection of X-Men; some from the ‘First Class Universe’ and some from the ‘Original Trilogy Universe’, all going forward together into 'X-Men Apocalypse' and 'X-Force' without having to worry about what has happened in the already released films…
Having set 'First Class' in the 1960's and part of 'Days of Future Past' in the 1970's it would make sense for the next films to be in the current day, especially if they want the new Fantastic Four to crossover at one point.
'X-Men Apocalypse' and 'X-Force' could follow on from the 'Origins/X1/X2/X3/Wolverine' timeline - after the actions of 'Days of Future Past' has reset the damage that is hinted at at the end of 'The Wolverine'...