24. Before (meaning ‘before that') follows an expression of time.
Last summer, I decided to go and visit the town that I had left eight years before. (NOT … ̶b̶e̶f̶o̶r̶e̶ ̶e̶i̶g̶h̶t̶ ̶y̶e̶a̶r̶s̶.)
I had already met her once, about three years before.
For more details, see Practical English Usage Third Edition 96.