If NULL
, then city populations from the 2010 census
(the most recent census) are returned. This parameter accepts a Date
object or a character string coercible to a Date
object, as well as
numeric values representing a year. If a year or date is used, then city
populations from the decennial census from 1790 to 2010 prior to
that year is returned. For example, 1805
or "1805-07-04"
would return city populations from the 1800 census.