Hmm, I guess the documentation is just a little confusing.

{
      "name": "scrollDomainStart", "init":"initialScrollStart",
      "on": [
        {
          "events": {"signal": "scrollTimer"},
          "update": "scrollRun ? scrollDomainStart + 60000 : scrollDomainStart"
        }
      ]
    }

init does work fine as the update is within the event handler rather than the properties of the signal.