Use item in item key

[Glide]

I have an item containing a file path, Key: some.log.file
I need to get the modification time of this file in an item.

So I naïvely tried to create an item with the Key: vfs.file.time[//some.log.file]
Of course it's complaining about the file not existing.

I tried enclosing //some.log.file between ``, "", {}, it doesn't seem to help.
How can this new item evaluate //some.log.file before executing the vfs.file.time function on it ?
Should I use a calculated or dependent item ? If yes, how ?

[Glide]

I finally used a single system.run item instead of 2 items, as it seems complicated to base one onto another.

system.run[echo $(($(date +%s) - $(date +%s -r `ls -td my-directory/* | head -n1`)))]

This gives me the age of the latest file on a directory