I'd be happier if you implemented the suggestion above and restore symmetry between `jd1` and `jd2` and explicitly use floats in the call to `iers_table.ut1_utc`.  In general (even if it is a bit of pain) the API should not make a strong distinction between JD1 and JD2.  There is no real performance penalty here and then the code itself will be crystal clear without even needing comments.