interval

http://environment.data.gov.uk/wales/bathing-waters/data/bathing-water-profile/ukl1100-39985/2015:1/intervals/interval

has beginning
2015 01 01 t00:00:00
http://reference.data.gov.uk/id/gregorian-instant/2015-01-01T00:00:00
in xSDDate time
2015-01-01T00:00:00