Comment by anonfunction

I had the same thing happen to me. From the manpage I gathered that units uses the definitions defined in /usr/share/misc/units.lib, by running cat /usr/share/misc/units.lib | grep light I found I only had lightyear and it's shortcut ly defined. I added lightsecond, and since milli prefix is already defined it worked a treat.

Here's the line you'll want to add:

lightsecond lightyear / 365.25 / 24 / 60 / 60