Legend: (federal) = federal holidays, (state) = state holidays, (religious) = religious holidays, (week) = weeklong holidays, (month) = monthlong holidays, (36) = Title 36 Observances and Ceremonies Bolded text indicates major holidays that are commonly celebrated by Americans, which often represent the major celebrations of the month. See also: Lists of holidays, Hallmark holidays, public holidays in the United States, Puerto Rico, United States Virgin Islands, New Jersey and New York.The international format yyyy-mm-dd or yyyymmdd is also accepted.
The most common relative dating method is stratigraphy.
Other methods include fluorine dating, nitrogen dating, association with bones of extinct fauna, association with certain pollen profiles, association with geological features such as beaches, terraces and river meanders, and the establishment of cultural seriations.
Because the pool of radioactive carbon in the atmosphere (a result of bombardment of nitrogen by neutrons from cosmic radiation) has not been constant through time, calibration curves based on dendrochronology (tree ring dating) and glacial ice cores, are now used to adjust radiocarbon years to calendrical years.
The development of Atomic Absorption Mass Spectrometry in recent years, a technique that allows one to count the individual atoms of 14C remaining in a sample instead of measuring the radioactive decay of the 14C, has considerably broadened the applicability of radiocarbon dating because it is now possible to date much smaller samples, as small as a grain of rice, for example.
Relative dating methods allow one to determine if an object is earlier than, later than, or contemporary with some other object.
It does not, however, allow one to independently assign an accurate estimation of the age of an object as expressed in years.
The earliest-known hominids in East Africa are often found in very specific stratigraphic contexts that have implications for their relative dating.
The majority of chronometric dating methods are radiometric, which means they involve measuring something related to radioactivity, for example radioactive decay.
In 2003, California Senate Bill 420 was introduced to regulate medical marijuana use, in deliberate reference to the status of 420 in marijuana culture.