If time changes with Speed, what is the difference in a second on a
fixed point in space, for examply the center of the universe,
compaired to a pixed point on earth which is rotating round the Milky
Way and which in turn is moving away from the center of the universe.

In Einsteinian relativity, there are no fixed points in space. No
unique center of the Universe. There are only relative distances &
velocities between pairs of reference systems. Given a pair of systems
with a relative velocity (v < c), there will be a Lorentz
transformation (the gamma factor) of time & length given by

gamma = 1/SQRT[1 - (v^2/c^2)]

That is all true, however, we do know how fast the earth is moving
with respect to the cosmic background radiation (CBR). This is as
close as we can come to defining a reference frame for the "center of
the universe". The earth is moving at approximately 370 km/s toward
the constellation Virgo. This is roughly one thousand times slower
than the speed of light, so the Fitzgerald contraction factor is
roughly:

1/sqrt(1 - 0.000001) ~ 1.000002

So we would observe a clock that is stationary in the rest frame of
the CBR ticking slower by about 2 parts in a million, or about one
minute per year. A very small effect but easily measurable with a good
quartz crystal clock as reference. Of course an observer stationary in
that frame would note that OUR clock is the one going slower. That's
the cool thing about relativity!