Thank you, Hélenè!
The sample form was exactly what was needed - I had tried combining conditional once(now()) before, but didn't get it to work. This is already a workable solution, though a native function would of course be easier to use.
I was hoping to get ISO8601 timestamps matching those generated by "start" and "end" , such as
2018-04-11T14:02:37.495+03
My calculation is therefore
if(${name} = '', '', once(format-date-time(now(), "%Y-%m-%eT%H:%M:%S.%3")))
... but the timezone is still missing, however, and format-date-time doesn't recognize the escapes Z or z. This isn't a huge problem, but is there some easy solution?
Thanks,
Sebastian