We should also set all existing countdowns to have the authorPHID as the edit policy, to preserve the old behavior (e.g., if only alincoln could edit a countdown before this change, only alincoln should be able to edit it after this change).

To do this, you can add a third patch like this:

UPDATE {$NAMESPACE}_countdown.countdown SET editPolicy = authorPHID WHERE editPolicy = '';

That will copy the authorPHID into the editPolicy for countdowns with no edit policy. User PHIDs as policies mean "just that user", so this will preserve the old policy.

You can delete this stuff now after modernizing with handleRequest().

You should read id out of the $request, using $request->getURIData('id'), and remove the $id property on the class. Generally, modern stuff should usually look something like:

final class SomethingWhateverController
  extends SomethingController {

  // Most controllers now have no properties. A few complex controllers still will.
  // Most controllers now have no willProcessRequest() method.

  public function handleRequest(AphrontRequest $request) {
    $viewer = $this->getViewer();

    // Read any URI properties from `$request` here, into local variables.
    $id = $request->getURIData('id');
    if ($id) {
      // ...

Overall, the goal of this change is just to make controllers a bit simpler -- before, like 50% of them had an $id property (or something similar) which was used in ~1 place and a willProcessRequest() method that did almost nothing. The new way isn't a big change, but gives us like 5 lines of code instead of 10, and is a bit simpler, especially for small controllers.

Minor, but prefer PhabricatorTime::getNow() to time() (see T6991).

There's a task for cleaning this up somewhere, but this way of doing things is a little out of date.

Instead, just call setValue($v_view) on the AphrontPolicyControl, like how every other control works. You'll also need to initialize $v_view and $v_edit near line 48, where other properties are initialized.

This change just makes policy handling more consistent, so the policy controls work like other controls do instead of in a special way.

Very minor, but you can appendControl() instead of appendChild() to get setUser() for free. That is, this:

id(new AphrontFormView())
  ->appendChild(
    id(new SomeControl())
      ->setUser($viewer))

...can be simplified slightly like this, which is equivalent:

id(new AphrontFormView())
  ->appendControl(
    id(new SomeControl()))

(One level too-indented?)

Prefer PhabricatorTime::getNow().

This should likely be getEpoch() or something simialr.

Not quite English.

To make this more general, it might be better structured as withEndDateInRange($start, $end) or similar, where either parameter can be null to indicate that you don't care (i.e., an open-ended range). You can look at PhabricatorWorkerTriggerQuery->withNextEventBetween() for an example of how to implement this. This isn't too important, but increases the generality of the query at little cost.

(Indent one off?)

Maybe we should consider removing this automatic capability now that there's a real edit policy, although I don't think it's too important and giving the author special privileges seems reasonable-ish.