It's the distant future, and humans have colonized Mars; and Mr E from England is visiting his Martian friend, Martin. In Martian English (off-shoot of the American English of the first settlers) Mr E tells Martin what he's been doing—he spent a very pleasant couple of days at some botanical gardens, where he saw a new Cactusoid (such as grow on Mars) that flowers every two days (of interest to Martin because he has a very dynamic garden, of which he is very proud).
......Although they're conversing in Martian English, Mr E could expect his "couple of days" to be taken by Martin to refer (quite literally, as far as Mr E is concerned) to two Earth days; but similarly, even though Mr E is from Earth, Martin might know that Martian Days (the literal meaning of "day" for him) were meant when they were talking about the new Cactusoid, because he would know why Mr E was mentioning it (i.e., because of what such a duration might mean in the context of Martin's garden).
......Similarly (re this post), whether or not the six days of Creation (in Genesis 1) are to be taken (literally) to be the durations of our days (or of the possibly slightly shorter days of whenever Genesis 1 was first written down, which would then have been literal), or to be such transcendental days (no less literally, were the Bible the word of God, in view of the topic of Genesis 1) as our mundane days would derive from—surely that depends upon what we think God's purpose was, in communicating such an account of Creation.
......If God wanted to inform us of superficial facts without us having to think very much (so as to avoid unreliability and elitism) then he could have arranged things so that it was the first of those options (although surely he would then have eliminated numerical contradictions from the Bible), but what would have been the point of such information? I've yet to investigate Genesis in any depth, but the first few chapters seem (at least superficially) to resemble parables more than history (e.g. why care who Cain married?).
......And the story of Noah's Ark (Genesis 6-9) presents a prima facie problem for any kind of literalism (e.g. perhaps it's prophetic!)—literalism could be saved in ways no weirder than those required to save Transubstantiation and Humean Supervenience (not to mention the Set and String theories of modern science, and their motivations), if we wanted it to be; but surely the issue is not so much what we want, as what we think is the purpose of the Bible (what God's motivation for Creation might most plausibly be).