When has "gravity" changed, or when have "measurements" of gravity been further refined? Please be specific.
Has it changed recently?
Gravity? Probably not. Measurements of it? They're refined quite often.
Speed of light changed recently? Is it now faster...slower?
See above. As far as we can tell, the speed of light has not changed. But we are continually refining our measurements of its speed.
Constantly refining the date of the planet tells me what?
That we're constantly improving our methods.
Is it right this time? I can answer that. No. It will be changed again and again and again.
Yes it will, but that doesn't mean it's "wrong" now. What you're forgetting is that scientific measurements aren't single numbers, they come with error estimates. The age of the Earth is known as X plus-or-minus some amount Y. (Actually, the value can exceed a deviation of Y, since that usually indicates a probabilistic range instead of an absolute limit, but that's too complex to get into here.)
The current (or past) measurements are only "wrong" if the true age of the Earth doesn't fall within the range (X-Y to X+Y). What happens with new, better measurements is that "Y" shrinks. You're concentrating on the fact that the new "X" is different than the old, while ignoring the fact that it still falls within the old (X-Y, X+Y) range. In other words, all the measurements are correct, but the later ones are more precise.
Speaking of the speed of light, some creationists (Setterfield, et al) try to "prove" that the speed of light is decreasing over time by taking various measurements of the speed of light (including some from hundreds of years ago) and drawing a sloping line through them. They "forget" (*cough*) to draw the appropriate error bars around each measurement. When that is done, it's obvious that all the measurements, even some of the very old ones, are all consistent with a constant, unchanging "true" speed of light. Except for some very ancient measurements, they were *all* correct, KEEPING IN MIND THE SIZE OF ERROR INHERENT IN EACH TYPE OF MEASUREMENT:
The bare "x" measurements were made without any estimate of their error range (which most likely was huge, given the antiquity of the measurements). The interesting thing to note is that once measuring methods became sufficiently good, around 1875, all of the measurements (plus their error range, don't forget) correctly measure the true speed of light, but that over time (and especially since around 1900), the precision of the measurements rapidly increase (i.e. the error bars get smaller and smaller until they're practically invisible) to the point where the measurements converge very closely on the "right" answer and there is little if any disagreement (or error range) on any measurement done post-1950. In fact, today's measurements of the speed of light are accurate to beyond the fourth decimal place: 299,792.4358 km/sec.
Isaac Asimov did a great job of explaining these concepts in his essay, "The Relativity of Wrong". His point is that measurements or theories are seldom entirely "right" or entirely "wrong". And how "wrong" (or "right") something is can be relative -- for example, Columbus' belief that the world was a sphere considerably smaller than its true size was wrong, but not as wrong as the belief that the world was flat. Similarly, Newton's laws of motion are "wrong" at speeds close to the speed of light, but they're still quite correct for 99.99+% of objects in the universe. And so on.
In short, you're confusing the difference between "accuracy" and "precision". Scientific measurements get refined as methods improved because the precision of the measurements is being improved (i.e., the error bars are constantly being trimmed down), not because the earlier measurements were "wrong" (that would be an accuracy problem).
I am not mocking and sneering. Just pointing out the fact that people put faith in something that changes so often over a short period of time.
Because we understand why it changes, and why that does not invalidate the methods or the results. Quite the contrary, in fact.