The use of radiometric dating based on the decaying away of radioactive ¹⁴C is the most useful technique for building sensible archaeological and climatic records over the last 50 thousand years.  However, this radiocarbon is produced from ¹⁴N by cosmic rays in the upper atmosphere, and their flux varies with time.  Consequently, the proportion of ¹⁴C in the environment varied in the past, and a radiocarbon age is not necessarily an age in calendar years “before present” (BP).  Even BP is confusing, because it isn’t “before now” but before 1950 when the first hydrogen bombs produced ¹⁴C.  The outcome is one of some confusion.  If dates were recorded in calendar years, whether BP or AD/BC everything would be clear.  But they aren’t.  Many authors give their dating as either ¹⁴C ages (BP) or calendar years (BP), and the two can be very different.  For instance, the date when the Younger Dryas glacial pulse began is 1000 calendar years older than its ¹⁴C age.    One reason for the dichotomy is that no agreed conversion existed until about 1998, particularly for the time before which annual growth rings in trees can be built into an unambiguous record, using modern trees and those preserved in ancient timber.  Bristlecone pines and other long-lived trees first gave an accepted conversion factor that went back around 6000 years.  That has been extended to about 26 ka by dating annually layered corals, stalagmites (speleothem) and sediments.  A way of going even further back is correlating large, world-wide events between their appearance in a record such as a marine sediment core, dated using ¹⁴C, and their appearance in a Greenland ice core, whose annual layering gives a calendar age.  However, further back in time less radioactive ¹⁴C remains to be measured and contamination by later carbon introduced by percolating water blurs the dating.  In September 2003 the 18^th International Radiocarbon Conference tried to clear the air (Bard E. et al. 2004.  A better radiocarbon clock.  Science, v. 303, p. 178-179).  The latest “official” calibration curve, (INTCAL04) goes back to 26 ka.  But beyond that there are 3 quite different candidates for calibration, the sea-floor sediment-ice core curve, one based on annually layered lake sediments in Japan, and one from speleothem in a submerged cave in the Bahamas.  For a vitally important archaeological find, such as the paintings in the Chauvet cave in France, the ¹⁴C date of 31ka could range from 33 to 38 ka in calendar years.  Dates for fossil occurrences of Neanderthal and the first fully human Europeans could overlap or be so different that neither had an influence on the other.  Everyone hopes that the sea-floor sediment-ice core curve can be validated by new results, thereby giving a common age framework to all dateable materials.