Can I do this with custom task rules:
When task A completes, update task B’s due date to be 3 days after task A’s completion date?
If I cannot do this with custom task rules, can I do it by building my own API integration using webhooks and the Asana API?
You can build this custom rule:
The triggering task (Task B in your example) will have its due date set to “3 days in the future”. Since the rule would always trigger on the date that the task is no longer blocked, which by definition is the same date its predecessor task (Task A) is marked complete, the due would meet your criteria of “3 days after task A’s completion date”.
The caveat is that the rule would apply to all dependent tasks in the project. If you only want it to apply to certain task(s), the standard way to do that in Asana is to use a custom field set to a specific value and then add another trigger condition to your rule of “Custom Field X is set to a value of Y (the specific value you’re using)”. Does that make sense?
If the above doesn’t work for your use case, you can also accomplish your goal using the Dynamic Duration workflow in our Flowsana integration. But if the above is workable for you, then you’ve got your solution.