[chaley]

Create a custom column 'Built from other columns' (a 'composite' column). Use the following as the template:

program:
	select(formats_modtimes('iso'), 'CBZ')

It could be that using a composite column will be too slow, given that it needs to read the disk for each book to get the modtime. You don't say how you use this data. If having it in book details is sufficient then hide the column in the book list, which will dramatically improve performace.

You can also use the Action Chains plugin to set the value of a date custom column to the result of the template on demand or when certain events occur. If you want to do this and need help then ask in the plugin's thread.

EDIT: you can also do it in bulk metadata search & replace using the above template as the source and the date custom column as the destination.

EDIT2: You can of course format the date using different format strings, for example

program:
	select(formats_modtimes('yyyy-MM-dd hh:mm:ss'), 'CBZ')