This method is only available on newer versions. The first available version (v2_6_3) is shown here.
sec_fraction()
Returns the fractional part of the second.
DateTime.new(2001,2,3,4,5,6.5).sec_fraction #=> (1/2)
static VALUE d_lite_sec_fraction(VALUE self) { get_d1(self); return m_sf_in_sec(dat); }