Is a vacation day counted as 24 hours or 8 hours to only cover the 8 work hours?

Location: Germany. Recently I got a bullshit explanation that a vacation day only covers 8 work hours, and if you work 12 hours, one must spend 1.5 vacation day to cover the 12 hours shift. This is the first time I ever heard this, and I have worked 12 hour shifts for near all my life. Never heard this. I always thought that a Vacation Day means a calendar day, 24 hours. The clue is in the name. I looked at law, and nowhere did I found this explanation, which to me sounds complete and utter bullshit.