XLSForm proposal to calculate a value when another value changes

WIP: discussion with @ln and @Xiphware:

type name label calculation when
text a Enter text
dateTime b now() ${a}
dateTime c Date of diagnostic now() ${a}
dateTime d Date of something ${a} empties value!
dateTime e ${a} empties value!
Change rules:
  • If there is no calculation a label is required unless it has when.
  • If it has a label, a body element is included in the XForms output.
  • An empty calculation with when, will reset the value to empty.