C = mod (A, B) returns the remainder left after dividing A by B, as a duration array of the size the two broadcast to. At least one of A and B must be a duration; the other may be a duration or a numeric array, which is read as a number of days, the unit a duration counts in.

The result takes its sign from B and is zero when B is, which is what distinguishes mod from rem: mod of an hour by $-1$ day differs from rem of the same. A remainder by a zero duration is A itself. The display format is taken from whichever operand is a duration, or from A when both are.

Source Code: duration