Difference in DATEs

Assume that x and y are of type DATE:

1. AGE( x, y ) < INTERVAL '...' works.

2. x < y + INTERVAL '...' works.

3. x - y < INTERVAL '...' doesn't work (but then, the minus operator is not 
defined in the manual for two DATE values).

Question:  Is the meaning of x - y well-defined?  That is, is there a 
definition that I can count on?

Not an important question, but I'm curious, since I like to write date 
differences in a style that makes readability/maintenance easy.

-- Dean