When a book is Added to a library:
- calibre stores the current timestamp (i.e. Now) in its timestamp (alias date)/Date custom column;
- the optional GetFileName plugin can copy the Date Modified of the format file used to create the book into a custom column.
The Date Modified of format files (EPUB, PDF etc) is also accessible via the
formats_modtimes template function. It can be used in a 'composite' custom column definition.
Date Created is peculiar to Windows… i.e. Linux and MacOS cannot provide it… hence there is no support for it in Calibre. It's often misunderstood anyway - it's the date the file object was created, not the date the content was first created. Hence the Date Created is often later than the Date Modified (which does refer to content).
Calibre has a read-only last_modified/Modified column that reflects the date a book's metadata last changed. Various actions can cause this column to be updated for multiple books in a library - e.g. addition or removal of a custom column. IMO the column is best ignored.
BR