Limitations and calibration: When Libby was first determining radiocarbon dates, he found that before 1000 BC his dates were earlier than calendar dates.
He had assumed that amounts of Carbon-14 in the atmosphere had remained constant through time.
In recognition of this problem archaeologists have developed regional reservoir correction rates based on ocean bottom topography, water temperature, coastline shape and paired samples of terrestrial and marine objects found together in an archaeological feature such as a hearth.
The Mayan calendar used 3114 BC as their reference.
More recently is the radiocarbon date of 1950 AD or before present, BP.
In a stratigraphical context objects closer to the surface are more recent in time relative to items deeper in the ground.
Although relative dating can work well in certain areas, several problems arise.