This is a hard one to sum up.
In the most basic sense: I want to set up a monument that communicates when it was made to people "Reading" it. But, I don't know who will read it, what language they will have, or even if/how they measure time. I also want it to be "Read" and not "Measured" - So this rules out isotopes. (Clarification: Direct measurements. We can carve in references to cyclical events, but not make it out of Unobtanium) I want it to be part of a story of events, so it'd be "At this point we did X. At this point before that, we did Y." and so on.
My current thoughts are star charts, but I'm not sure how precise these are. A year, a decade, a century? What other ways are there to state an "absolute" time?
Updates and Clarifications
Humans are building the monolith, and it is meant to be ready by humans. The purpose of the monument is to communicate a general pre-history (Relative to the monolith), and anchor "new" history to this pre-history. Why? Well, we've fled a cataclysm and found a new planet to settle - but we don't have the industry, technology, or other support to set up with proper, or much of any, technology. This means we don't really have the time or energy to spare in hunting down special elements - We've used the travel time to plan and design this monolith, so now it's a matter of carving it out. That can be done with hand tools, without using up valuable, diminishing resources.
We can't guarantee that language will stay the same, or that we'll even be able to keep writing around consistently - We come from a pretty technologically advanced place, so even something as "basic" as paper will take a significant amount of time to re-create. Especially using alien plants. And even when we figure out how to make it, we'll have to figure out how to harvest materials, process them, build the machinery and tools to do so... It's very time consuming even if you know exactly how to do it.
We can't guarantee that timekeeping will stay the same. Honestly, it's more of a guarantee that it won't. Calendars are useful for tracking seasons, so it's almost certain to be adjusted, tweaked, fiddled with, and probably completely replaced with something more accurate to the new world.
Relying on some form of oral history on how to translate the monolith will help, but it's primarily designed to communicate history for when it becomes relevant - When you're trying to find out how to build a civilization a few centuries prior to yours, without any preparation, your primary concern is surviving. When humanity has established itself, it can go back to the monolith and figure out what happened before.