I've got a spreadsheet in Google Drive and one column is meant for the date the entry is made. Is there a button to do this? I'm aware of the functions =NOW() and =TODAY() but their description says "the value is updated when the document recalculates". However I reopened the document a few minutes latter and it still had the same time. Why is this?

It would be even easier that when I start typing in a new row the column for the date auto-populates.