Right-click the bookmark and hit properties, that shows the date you saved it.

My uncle ran a repair shop in Parma for 22 years and he told me the same story over and over: guys would drag in a deck that sat in a garage since 1994, he'd flip it on to test it, and the belt would turn to goo or the capstan would chew whatever tape was inside. He started keeping a junk tape in every machine just so nobody's real stuff got wrecked on the first spin. That's the thing about old gear, it's not the machine that's the problem, it's that rubber and grease dry out and turn mean when nobody's looking. You didn't do anything wrong by being nice, you just skipped the one step every old deck needs, which is a warm up run with something you don't care about. As for the tape, a clean snap at a splice point can sometimes be fixed with splicing tape if he still has both halves and the magnetic coating didn't get scraped off. If the break is in the middle of a song though, you'll hear a tiny drop out forever, and he'll notice it every single time he plays it. Did he keep the pieces or did they end up in the trash right there?