Function to convert certain ISO 8601-compliant formats [date(yyyy-mm-dd) or dateTime] to epoch/Unix time

date-to-epoch-time.xqm

module namespace epoch = "https://gist.github.com/WaxCylinderRevival/ns/xquery/date-to-epoch-time";

declare function epoch:date-to-epoch-time
  ( $dateString as xs:string? )  as xs:decimal? {   
    if (empty($dateString))
    then ()
    else 
      if (matches($dateString, '^\d{4}-\d{2}-\d{2}$'))
      then (xs:dateTime(xs:date($dateString)) - xs:dateTime("1970-01-01T00:00:00-00:00")) div xs:dayTimeDuration('PT1S')
      else
        (xs:dateTime($dateString) - xs:dateTime("1970-01-01T00:00:00-00:00")) div xs:dayTimeDuration('PT1S')
 };