How to write time duration correctly

Question

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?

Answer 1

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.