Fill time and date fields if empty with current value

[jmvanel]

Implementation in Scala.js in project form_js ( quite similar to the GPS code )

For now, the relevant RDF property is dcterms:date ( cf http://deduction.github.io )

NOTE: anyway, the time stamp of a triple group (<s> <p> ?O ) is recorded by the application, see trait TimeSeries; but this is for cases when the time stamp needs to be explicit in the form, and possibly edited by user.