@chaley: I assume he meant "I tried it here and it didn't work.
@dunhill: The following is working here:
Code:
program:
days_between(field('last_modified'),field('#last_seen'));
I only have the one date custom column in the library I have here. But, it worked with the Date a Published date columns as well (once I typed their names in correctly

.
And as an aside, I absolutely hate the fact that changing the template for a column changes the modified date for all books in the library. The same goes for adding a new column. Or even changing the column header. I've handled that in the source at home, but, I don't run from source on my work machine.