> "Back when I was a young man, people always wore hats."
> "Back in the day" is its own expression, meaning "Back at a previous time or circumstance"
- "Back in the day, you never saw boys disrespecting girls like they do now."
- "Back in the day, I was the most famous rodeo clown in the circuit."
The existence of that expression ("Back when") may provide some legitimacy to "A few days back, I happened to bump into Hensen."
I think "a few days back" might be more common in some regions of the US (such as down South, or maybe out West) than in other places. I think it's rural anyway.