Even if there were retroactive changes for some reason, generally when storing a time in the past, you want to know the actual time it occurred, so a conversion

Even if there were retroactive changes for some reason, generally when storing a time in the past, you want to know the actual time it occurred, so a conversion to UTC when storing the time stamp is unambiguous. For future dates, you are often trying to represent "wall time", not a specific point in time, which is why storing as UTC is the wrong representation - it's something that maps to the information you want, but the mapping can change between storage and retrieval.