The date:sum function adds a set of durations together. The string values of the nodes in the node set passed as an argument are interpreted as durations and added together as if using the date:add-duration function.

The string values of the nodes in the node set passed as the argument must be in the format defined for xs:duration in [3.2.6 duration] of [XML Schema Part 2: Datatypes]. If any of the string values of these nodes are not in this format, or if the node set is empty, the function returns an empty string ('').

The result is a string in the format defined for xs:duration in [3.2.6 duration] of [XML Schema Part 2: Datatypes].

The durations can be summed by summing the numbers given for each of the components in the durations.