> I tried to use the command
> Subst on edit Last Edit = $entry time
> but it gives a "-" for the Last Edit value when I edit an entry. 
> 
> I think this is because $entry time is not a variable supported by Subst. Can you add that support, or else tell me if you know a better way to go about 
> doing what I'm attempting? Is there perhaps a way to globally process a group of entries in a logbook and set one attribute's value to be equal to 
> another's?

I tried to reproduce what you said and it worked fine for me. I used:

[demo]
Attributes = Author, Type, Last Edit, Subject
Subst on edit Last Edit = $entry time

And got correctly the entry time copied to the "last edit" field when I changed old entries. I used version V.7.3-2104.