Examples:

• Back in the day, this whole area was under water.
• Back in the day, this used to be my beat.
• You know, back in the day you were pretty close.
• Bet it was nice and shiny, back in the day.
• Back in the day, bad poker players were called fish.
• Done a few of those trips back in the day.
• Ryan used to work for them back in the day.
• Kind of reminds me of us back in the day.
• Did you know him back in the day, too?
• It was all over the news back in the day.