An S4 class to represent the (Gregorian) BP era.
See also
Other classes:
AD-class,
BC-class,
BCE-class,
CE-class,
GregorianCalendar-class,
JulianCalendar-class,
RataDie-class,
TimeScale-class,
TimeSeries-class,
b2k-class
Other gregorian era classes:
AD-class,
BC-class,
BCE-class,
CE-class,
GregorianCalendar-class,
b2k-class