Can you just do:
```
self_tai = self.tai
other_tai = other.tai
return (self_tai.jd1 - other_tai.jd1 + self_tai.jd2 - other_tai.jd2)
```
I *think* this should work, but maybe there is something I'm missing.  Using those properties instead of the internal `_time` attributes does the `shaped_like_input` already, so you could remove the method below and clean up the code a lot.  But like I said maybe there was something I didn't think of.