Table 3. Guidelines used for converting the introduced date given in the reference into a whole number.

Date given	Grouped (converted) date	Rule
1912	1912	Use the four digit number as given
c.1890	1890	Use the four digit number given
1777–1778	1777	Use the earliest date in the range
1930–1940	1935	Use the midpoint of the range
C18th	1750	Use the midpoint of the century
early C18th	1710	Use the date 10 years into the century
mid C18th	1750	Use the midpoint of the century
late C18th	1790	Use the date 10 years before the end of the century
1800s	1850	Use the midpoint of the century
c.1800s	1850	Use the midpoint of the century
1990s	1995	Use the midpoint of the decade
early 1700s	1710	Use the date 10 years into the century
mid 1700s	1750	Use the midpoint of the century
late 1700s	1790	Use the date 10 years before the end of the century
early 1990s	1991	Use the first year of the decade
mid 1990s	1995	Use the midpoint of the decade
late 1990s	1999	Use the last year before the end of the decade
1980s–1990s	1990	Use the midpoint of the two decades
<1965	1964	Use the date immediately before the date given
>1970	1971	Use the date immediately after the date given