I tend to go for "day" in the Genesis 1 account to be the period between dawn and dusk (which is, to an extent, hinted at in the "evening and morning" references). That is, it is the period of time when, without modern electric lighting, it was actually possible to do work.
I'd also say that the Genesis 1 account is not a literal historical account of creation so I feel no need to try and reinterpret the meaning of the word "day" to fit into any particular scientific description.
Alan