If all of time were compressed (the magic was executed at some time,no matter) it means all eras feels it and not only era B (see the meaning some replies earlier).
Definitely.

1-Let me give an example.
Lets suppose we have two eras C and D.And two instances of the progression of time compression a and b.b>a and D>C.b is the instance where time is fully compressed.
WE also have D-C =b-a.
Eras C and D will evolve with time.When time goes from a to b Era D will not be D anymore but will be an era E.
Should we conclude that time compression will not be feel on era D?
No,because era C will become era D.Of course there isnīt any instance greater than b because time stops but i showed it didnīt matter because there are always eras before others.One can wonder though if this means that there is many states of present .One in which era D feels time compression and another in which it didnīt feel.But...
Not entirely sure I understand everything you're saying here. Yes, C will become D, but era D will always be era D. Well, I guess that depends if you think TC limits itself to era D exactly, or if you're talking about all of time being compressed.

And it also depends on whether or not you think time is static or dynamic. Too many "if's" for me to be able to understand your conclusions, so you'll have to go into greater detail here.

2-It didnīt make sense to say that all eras get compressed to a "particular time"
If all times get compressed it means they get compressed without a time referencial.
They just get compressed.If all times get compressed (sucked like a whirl of water) to 1000 they also get compressed to 1630 or any other time.And...
If the line of time were infinite then it wouldn't make sense, true. If it's finite, it does make sense to have a referencial point(namely the point where it is cast) all until time is fully compressed, because then the referencial point would be the same as every other point on the entire line of time.

3-You canīt compress an infinite time line in a finite time line.
If you take a line segment and reduce it you can do it in a finite time.
But not an infinite line.
True. Although, I wonder what would happen if TC moved at an infinitely fast speed? Would the infinities cancel each other out making it possible? Too tired to think it through now though...

Of course we don't know if the line of time is finite or infinite so no point using that factor in arguments.

So I conclude there isnīt any progression in time compression.
Ultimecia just compress time instantly.It was a magical and instantaneous change of state.
I'm sorry, but your conclusion is impossible. As I quoted earlier, we are told that no one but Ultimecia can live in TC, but if TC was an instantanous event, it would be impossible to play the game, as Squall and co would be inside a fully time compressed world already from the start of the game.

Since this is obviously not the case, we can conclude that TC works at a finite speed, and is never fully completed.