I've seen the standard epoch described as both J2000 and J2000.0. Is there any difference between the two designations?
They're identical. See https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000 "J2000 = JD 2451545.0 (TT)" - PM 2Ring
They are the same. It is possible for an epoch ending with a fractional amount, which is why some people put the .0 there. E.g. the Hipparcos catalog's epoch is J1991.25, which indicates a fraction of a year. - Greg Miller