I think it was implied that star platinum had the time stop ability the whole time. When jotaro confronted dio, he quickly figured out that their stands were very similar in terms of speed, range, strength, size and what not. And once kakyoin figured the world's secret ability, it dawned on jotaro that star platinum might have a similar skill.
Why did star platinum get weaker? Star platinum may have gotten slightly weaker overall. However, the main reason josuke got the upper hand is because he caught jotaro off guard and didn’t give him much of a chance to prepare himself.
Not only that, but crazy diamond’s strength, speed, and precision are very close to that of star platinum. Same reason why star platinum doesn't stop time all the time. From looking at stuff it seems instead of being able to train his ability to stop time, he actually slowly loses it.
Seems that the more he stops time, the worse he gets at it, due to him being a person and not being able to handle stopped time. Sta platinum's punch by the normal punch of a human and you have 500,189,520. This means star platinum is 500,189,520x stronger than an average human.
Now, on to dragon ball equation. The average human has scaled according to raditz's scouter as a. At the start of part 6, star platinum has an a in durability.
After pucci snags the discs and jolyne returns the stand one, jotaro has no memory left. There's a chapter where the speedwagon foundation comments that jotaro can hardly take a shower without help. The fact of growing weaker and older over time was literally the reason that straits used the stone mask to kick off part 2's plot.
He wanted to remain young and powerful forever and so trained for years in hamon,but when he realised it was only delaying the inevitable he became a vampire to gain eternal immortality. Back when it was just star platinum, the stat referred to its physical defense or how long it could keep punching, since that was its primary ability. Seriously if anything, star platinum should've gotten stronger in the time skip.
Stands get weaker as they get older. This is confirmed in part 5. And if jotaro had actually been using sptw, then he would be almost as strong as part 3 sp since 40 isnt thaaaaaat old.
