It looks like xsd:duration uses duration notation from ISO8601. The rules for combining them or designating recurring intervals are in ISO8601 section 4.4-4.5 [

It looks like xsd:duration uses duration notation from ISO8601. The rules for combining them or designating recurring intervals are in ISO8601 section 4.4-4.5 [1]. Unfortunately, awareness and library support for ISO8601 interval notation are very sparse and most people/orgs just roll their own. [1] https://www.loc.gov/standards/datetime/iso-tc154-wg5_n0038_iso_wd_8601-1_2016-02-16.pdf https://www.loc.gov/standards/datetime/iso-tc154-wg5_n0038_i...