[davidkuhta]

Hat tip to @nfriedly from back in 2011:

FYI units on OS X doesn't recognize millilightseconds, but you can do this:

  You have: 3 lightyears / 365 / 24 / 60 / 60 / 1000

  You want: miles

   * 559.21802

  / 0.0017882113

[jxy]

    % units
    586 units, 56 prefixes
    You have: 3 lightyear*millisec/year
    You want: mile
     * 558.84719
     / 0.0017893979

or you can patch up the library file

    /usr/share/misc/units.lib

[dredmorbius]

GNU units is available through Homebrew, and does.

[dheera]

You can also type "3/1000 lightseconds to miles" in Google.

[microcolonel]

GNU Units is spectacular.