You are right about people using it in a variety of ways. The important aspects of a construct is the arbitrary nature that comes with it. 13 months or 12. Etc.. But time is the largest of those constructs.
Time does not exist. Time is an explanation to describe concepts more easily. If time was real backwards time would actually be possible. But since time isn't real only forward time is possible.

@Biosteelman Time is a measurement of motion, a useful relationship that we have devised. I think I get what you're saying, but the measurement now exists, is real and repeatable. The units (seconds, minutes, hours, days, months, and years) we have chosen somewhat arbitrarily, but they are very useful for our lives (except maybe months). Without the measurement, much of science, the ability to effectively communicate cooking recipes, and the like, would be difficult.

Back to "human constructs." I would say, by that definition, that most of our concepts, above first level concepts (concepts of entities, like dog, cat, tree, chair, etc.), are "human constructs." But they are valuable in that they allow us to hold integrated knowledge in our minds efficiently. Not all concepts are valid, but I have found most to be.

@sfvpool correct. Understanding the difference between a construct and reality allows us to better develop and control our surroundings.

Ummm, not every one celebrates Christmas. The whole planet celebrates New Year's Day. It's a celebration based purely on the passage of time.

@sewchick57 The whole plant - really, you have a source? B/c I don't see the Chinese celebrating - at least not on the same day. I also don't think many poor in sourth/central america nor in africa are all hyped up over it. I also know that many people from India don't celebrate, although some do.
Just my experiences when I was in other countries or worked with foreigners.