There is always something to measure against, time is just a measurement from one point to another, you just need something that repeats itself more or less constantly such as an orbiting planet, if you want to measure it.
But in regards to the bible, I don't really think it makes sense to view the days as being different lengths compared to today. Unless Earth was in a completely different orbit back then, which I highly doubt. So a day in the creation story would be the same as today.
The same goes for a foot, it is just a name for a fixed length, but you could just take a random stick and then say that everything is measured based on that length. So a foot obviously didn't exist without humans