This is completely incorrect. Massless particles like photons don't have a rest frame so any statement about their "concepts" are borderline meaningless. Saying "time doesn't exist without mass" makes no sense. A universe with only massless particles would still time-evolve.I think time is a construct that helps explain causality (sequences of events), so we can understand them easier. It is clearly relative and doesn't exist without mass. Massless particles like photons have no concept of time. That is another way of saying they exist in all time simultaneously.
Time is certainly real. 100 million years ago the hour hadn't been invented and 100 million years from now the hour, if humans are still around and using SI units, will have the same meaning. The modern SI hour is defined in terms of fundamental physical laws, not 1/24th of a day.It's not a reach to go from that to time isn't real. It's a property of reality rather than reality...a way of describing what we observe rather than what actually is true. The 2023 vs 2003 is nonsense because neither is real, they are manmade constructs like noon or Monday. We have months because of the lunar 30 day orbit. It's not exact its approximate. Similarly, all our measurements of time are guesses. 100 million years ago what we would call an hour (1/24 of the day cycle) would be different amount of time than it is today or a 100 million years from now.
This is wrong. Again, making statements about observers travelling at the speed of light is almost certainly nonsense (since observers can't travel at the speed of light). The speed of light being constant has no bearing on whether or not time is a constant (spoiler alert: time is not constant).BUT, at some point, there is an absolute time. At the speed of light, time doesn't exist and speed is distance over time. Since the speed of light is a constant, at some level, time is a constant too. This could be explained by saying, there is no such thing as time.