The "Accessed" column/field is filled with the current date/time when an item is added from the web. You can add/modify it at any later point.
Items that weren't imported from the Web don't have accessed dates: that could be manual entry or import of metadata.
If you just want to keep track of when you added (or last modified) items, you want the Date Added/Date Modified columns (which do what their names suggest and can't be changed)
Thanks Adam -- this is helpful. Just for further clarification -- for an item imported from the web -- the accessed date will be the date of import. Do I understand correctly that date will not change if I simply open the item or drag it to another folder? Will it change if I modify the entry in some way -- e.g., add a tag or update a link? (The reason I am asking is because I plan on importing data from a Zotero export file -- so the Added date will be the date of the import. I am hoping to use the Accessed date as an alternative method of knowing when the item was actually added from the web.)
Items that weren't imported from the Web don't have accessed dates: that could be manual entry or import of metadata.
If you just want to keep track of when you added (or last modified) items, you want the Date Added/Date Modified columns (which do what their names suggest and can't be changed)
(The reason I am asking is because I plan on importing data from a Zotero export file -- so the Added date will be the date of the import. I am hoping to use the Accessed date as an alternative method of knowing when the item was actually added from the web.)