A Calibre plugin to record when a book is viewed
To use, first create a custom column in Calibre with the name #lastopened and the datatype Date. The date format is up to you.
Then add this plugin's action to whatever menu you like. The default keyboard shortcut for it is R.
Triggering the action will open the selected book or books in the default viewer, and add the current date and time to the custom column.