Bignum referenced in Time documentation
Since #12005 Bignum has been unified with Fixnum into Integer class, but it is still referenced in Time documentation.
Since Ruby 1.9.2, Time implementation uses a signed 63 bit integer,
Bignum or Rational.
The integer is a number of nanoseconds since the Epoch which can
represent 1823-11-12 to 2116-02-20.
When Bignum or Rational is used (before 1823, after 2116, under
nanosecond), Time works slower as when integer is used.