In six days the Lord made heaven and earth

8 “Remember the Sabbath day, to keep it holy. 9 Six days you shall labor, and do all your work, 10 but the seventh day is a Sabbath to the Lord your God. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates. 11 For in six days the Lord made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the Lord blessed the Sabbath day and made it holy (Exod 20:8-11).

This text often crops up in debates over the days of Gen 1. Appeal is made to this text to establish two related points. The days in Exodus 20:8-11 are 24-hour days. And the days are consecutive. By parity of argument, so are the days of Gen 1. An uninterrupted sequence of 7 24-hour days. This is called the calendar-day interpretation. 

There's obviously some merit to this argument from analogy. After all, this isn't just a reader's inference. The text itself invites that comparison. 

However, there's also a danger of a modern reader unconsciously recasting the issue in provincial and anachronistic terms:

i) Ancient Israelites didn't think in terms of 24-hour days. That represents clock time. They didn't have clocks.

ii) Likewise, subdividing a day into 24 equal units of time is artificial rather than natural. 

iii) There's an equivocation about the "day." It's not as if Israelites labored 24 hours a day, 6 days a week, before knocking off work. The workday actually stands for daylight hours. As a matter of fact, you took breaks between one day and the next day. You labored during daylight hours, and rested at night. 

You were largely dependent on sunlight to be able to see. That could be supplemented a bit by firelight–or moonlight (a full moon on a clear night). 

Folks who traveled by foot had to allow for enough time to arrive at their destination before they ran out of light. 

iv) Moreover, day and night aren't like flipping a light switch, where it becomes instantly light at sunrise and instantly dark at sunset. First light precedes sunrise. Especially on a clear day, the sky brightens well before sunup, and darkens sometime after sundown. So the boundaries between day and night are fuzzy.

v) In addition, unlike clock time, which cuts across the natural divisions of day and night, sunrise and sunset undergo seasonal change. They vary every day. The daylight hours lengthen in the summer and shorten in the winter. 

vi) Not only is this sensitive to the temporal orientation (i.e. time of year), but spatial orientation (latitude). If you live in Alaska, there are times of the year when you have continuous sunlight, and times of the year when you have continuous night. When the sun never dips below the horizon or rises above the horizon. One day blends into another day. One long day. A "day" that's longer than a week. So the duration of a day is not invariable. It depends on where you live, as well as when you live. So the duration of a "day" is variable both internally and externally. When a day naturally begins or ends is quite different from clock time or calendar time. 

Of course, the Decalogue was originally addressed to people in the Mideast, but imagine applying it to someone living near the North Pole before the advent of electrical lighting. And Christian missionaries would have to make allowance for that contingency. Exod 20 does, indeed, draw a parallel between the ordinary workweek and the creation week. But the ordinary workweek doesn't necessarily map onto the creation week the way some modern readers routinely assume.