A time-independent level of C14 activity for the past is assumed in the measurement of a CRA.
There you will find the date when the event actually happened if it is different.
Thales - Static Electricity, Geometry, Surveying, Scientific Method 530 B. Pythagoras - Mathematics, Divine Proportion, Number Theory 500 B.
Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced.
Amongst accelerator laboratories there has been mooted the theoretical possibility of extended range dating to 75 000 yr , at present this seems difficult to attain because of the problems in accurately differentiating between ions that mimic the mass and charge characteristics of the C14 atom.
This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.