For example, can we use the half life of Polonium or Uranium to measure time and produce something similar to a calendar?
You basically are proposing to take metrology back to the 19th century/early 20th century on Earth when international standard units of measurement were defined by physical artifacts.
Suppose you are employed in the U.S.A., and you have a meter stick on your desk. How do you know it really is one meter long? Well, your employer hires a lab to come 'round every year and check it against their meter stick. But how do they know that their stick is accurate?
How it used to work was, every so often they went to some regional authority that keept a meter stick in a vault... And every so often the regional authority took their meter stick to the national lab in Washington D.C.,... And it was possible, at least in theory, that with great ceremony, and at great expense, the national meter stick could be carried to Paris France where it would be compared to the the one, authoritative physical artifact—a bar of platinum-iridium alloy,in a heavily guarded vault in the basement of the BIPM, bearing two scratches on its surface that, at a certain specific temperature, were defined to be exactly one meter apart.
It's much easier today. The meter is defined by a simple physical law, and anybody anywhere who is skilled in the art can build their own apparatus to calibrate their own meter stick without needing to compare theirs to anybody else's stick.
You are proposing a physical artifact—a lump of uranium—to define time. The only way for anybody to know whether their lump of uranium has exactly the same activity as the official standard lump is to take theirs to the lab where the official lump is stored and compare them.
Traveling between stars requires relativistic speeds over long periods of time, and your rate of radioactive decay is effected by how fast you are moving due to spacetime dilation. If you are using any currently understood method of propulsion at 0.9 C for 10 years to reach your destination, then turn around and come home again at 0.9 C, then by the time you get home, your clock will be about 11.3 years out of sync. At that point your material ratio would be wrong, and your half life would be permanently shorter than Earth's rendering the clock pretty much useless because there is no way to resync them. You are better off using an atomic clock, and linking it up with a computer that can be programmed to compensate for spacetime dilation.
What time is it? I am writing this at about 10:30. By telling you that and cross referencing when this was posted you can roughly guess where I am longitudinally on planet earth. So even on the same planet, there is no definitive clock.
Many computers use milliseconds since Jan 1 1970, but this is only for internal math, and this always gets converted to a local time zone.
Using radioactive decay, good timing instruments, redundancy, and clever math you can get accurate time, but it isn’t that important. If you can’t teleport between planets, then the exact time doesn’t matter, if you can teleport then you can just have the government teleport things at constant intervals to keep time.
So while it is possible, it is more likely that local standards will eventually overtake your centralized system and people will just convert between those.