"reliably dated" is correct and usable in written English.
You can use it when you want to express something that has been dated accurately and precisely. For example, "The fossil is reliably dated to have been around 130 million years old."
I don't mind being reliably dated to my fifties.
These images, however, could not be reliably dated.
The oldest such tools reliably dated in Africa had been only 25,000 years old.
Dr. David J. Helfand, a Columbia University specialist in pulsar research, said that until now, scientists knew of one reliably dated pulsar, the one in the Crab Nebula.
But those dates come from the youngest reliably dated fossils that have been found, and who is to say there aren't even younger fossils out there?
What previously were the oldest modern human fossils in Europe are only now being reliably dated, usually at around 30,000 years old.
Geoscientists can reliably date igneous rocks of any age by measuring what percentage of their radioactive isotopes has decayed since the rock was molten.
It was not feasible to use last menstrual period (LMP) to estimate gestational age in this cohort because the mothers could not reliably date their LMP.
Residual traces of arsenic in hair and nail samples may confirm arsenic exposure but can be subject to external contamination and cannot reliably date time of exposure (Hindmarch 2002).
Reviewing other Neanderthal dates ascertained with the new ultrafiltration method, Dr. Higham sees an emerging pattern that no European Neanderthal site can reliably be dated to less than 39,000 years ago.
More reliably it is dated to the 1940s or '50s, when King Zahir Shah was said to have bequeathed it to the state.
