Can't get timing dialed back to tdc

AJ/FormS said

Moving the drive gear accomplishes little to nothing,cuz, you just reclock the point cam, the canister moves to a new location, but way too far.I tried this years ago, to no good success. There just was not enough slots to achieve the goal, on my application.
IMO;you really have to prove the TDC mark first, then lengthen the slots as may be required.

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How do I prove the TDC mark? Im curious,why would that change and how likely is that to happen? When I rotate the harmonic balancer timing Mark to line up with tdc the rotor on the distibutor is exactly on the number one position as it should be. Why would tdc drift to the point that you couldn't dial the distributor back to tdc on the timing Mark? I've never seen this happen on any of the cars I've owned and all are much older than 1970. I know timing gears can supposedly jump a tooth,but that isnt that common of an occurance is it? That mostly happens on high mile engines. Our cars don't have nylon timing years like GM's do they? Wouldn't it seem more likely someone pulled out the distibutor and put it back in slightly off since it was like that when I bought it? Sorry for all the questions. Just wondering. I've never had this issue before.
Has your car ever gone this out of time that you couldn't adjust it back? There is only one timing Mark stamped into the balancer right? I didn't see another.