14C Laboratorium Datowań Bezwzględnych

Determination of the relation between the conventional radiocarbon age and the calendar age is the object of the calibration of the radiocarbon scale of time. The calibration is made on the basis of the calibration curve outlined by the 14C measurements of samples of a known age (mainly tree rings dated dendrochronologically). The so-called calibrated age, produced from the calibration, is the best approximation of the calendar age of the sample. The calibrated age is given as a range, which contains the calendar age of the sample with the probability of 95% (or 68%).

If you want to calibrate the conventional date you could use the program of the authorship of prof. Adam Walanus:
http://www.adamwalanus.pl/Kalib14C.html