N = datenum (D) returns a double array of the same size as D holding the length of each duration in days, including any fractional part. NaN durations give NaN.

A duration measures elapsed time rather than naming a point in it, so the result is a count of days and not a serial date number: adding it to a serial date number shifts that date by the duration. This is the same value days returns, under the name the older date functions use.

Source Code: duration