What does the app consider a "day"?

I'm aiming to get the 30k steps in a day achievement and was wondering if the app considered a day as in a calendar day, or if it's 30k steps in 24 hours?

30k steps comes out to about 15 miles. If the app goes off 24 hours, I was considering taking a backpacking trip some weekend and hiking half in an afternoon, stopping for the night, and doing the rest in the morning. But if it goes by calendar, I'd have to adjust my plans.