Fixed Time Span
A time span is fixed if it possesses the special property that its duration is independent of the assumed context.
As an example of a fixed span, “one hour” always, assuming a setting without leap seconds, has a duration of 60 min. To see that not all spans are fixed, consider “one month,” which is a prime example of a variable span in the Gregorian calendar. The duration of this span may be any of 28, 29, 30, and 31 days, depending on the context, i.e., the specific month.