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Within a user manual, I need to convey the maximum time period allowed as "365 days 23 hours 59 minutes 59 seconds" - are commas expected between each component? Is the word "and" expected before the last component regardless of the time period name?

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Are you following any particular style guides or internal style sheets? Your answer may be in those. – Neil Fein Oct 18 '12 at 17:28

I would use commas between each component, and use "and" only if the last component is seconds. I learned way back in intermediate school that "and" is only used before fractions (so 10,247 is said "ten thousand, two hundred forty-seven" but 10 7/8 is said "ten and seven-eighths"). I would call seconds the smallest "lay person" time interval — if you're getting into fractions of a second, you've moved beyond regular timekeeping and are now talking science, so it would be formatted differently.

So using your example:

The maximum time period allowed is 365 days, 23 hours, 59 minutes, and 59 seconds, so any longer than that is a problem.

BUT if your time were shorter, it would be written:

The maximum time period allowed is 365 days, 23 hours, 59 minutes, so any longer than that is a problem.

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Not sure about your last sentence. I understand your math approach (fraction), but shouldn't here the linguistic approach be considered? If you see it as an enumeration than an "and" before the last item is the standard way to do it (so not mandatory). Besides that I will never get used to the English habit putting commas before "and" and "or". Even we do not do that (at least not in enumerations). – John Smithers Oct 18 '12 at 15:13
I admit it does sound a little odd, but it also sounds odd to say "ten thousand, two hundred forty-seven" rather than "ten thousand, two hundred and forty-seven." In math, the "and" is a signifier meaning "fraction," so despite the linguistics, I'm sticking with that precedent. – Lauren Ipsum Oct 18 '12 at 23:54
And don't you knock my Oxford comma, dammit. I will fight to the death for the comma before "and." :) – Lauren Ipsum Oct 18 '12 at 23:55

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