This somehow breaks the symmetry in a weird way.  What about leaving both the defaults at `None` and then doing:
```
if jd1 is None and jd2 is None:
    utc = self.utc
    jd1 = utc.jd1
    jd2 = utc.jd2
else:
    if jd1 is None:
        jd1 = 0.0
    if jd2 is None:
        jd2 = 0.0
```