It means: fakeyear = Math.floor((realyear - 1000) / 10) + 1900 or equivalently approxrealyear = (fakeyear - 1900) * 10 + 1000 except that 1791 is repres

It means: fakeyear = Math.floor((realyear - 1000) / 10) + 1900 or equivalently approxrealyear = (fakeyear - 1900) * 10 + 1000 except that 1791 is represented as 1971 as a special case because it's a nice anagram (it should have been 1979).