I don't disagree with you that it's a construct, but it's a construct based on the observations of the known universe and the laws that apply. It's a system developed over the past at least 20,000 years, and has been constantly refined from counting the phases of the moon in the month to calculating Planck Time. Measuring time is a human construct because people needed to come up with a system to explain the movements of the sun, and moon, and universe. It's not arbitrary; it's quantifiable, and repeatable, and most importantly, predictable. I could claim that it was midday every time I saw a robin land in the oak tree outside my window, and that's a construct too, but it doesn't have any of the characteristics that the actual measurement of time has. That's why I made the joke; because it seems like people are becoming more prone to defining reality by how they feel, rather than what reality is. DST doesn't change the way time works; you can absolutely claim that it's midday at 5:30 AM, but your construct in no way changes that midday is the point where the sun is at its highest point in the sky. Now, you could call that event "Snarglelump" if you want, but whatever term you use, it has to describe that exact position of the sun, or it's meaningless. We agree as a species that time is measured the way it is, not because of some agreement between nations or society, , but because it accurately explains the movements of the planet and moon and celestial events to the point that it becomes predictable, quantifiable information. Someone could come along with a brand new system of measuring time, and unless it satisfied those requirements, it would be meaningless. We've abandoned multiple calendars over the millennia for precisely that reason; they didn't adequately explain the universe. Time doesn't exist because we measure it; we measure it because Time exists.
"No matter where you go, there you are."