Hi Lija,
It sounds like you need the automation to run on a schedule and add new tasks for any Documents where "Next Update" is equal to today.
You should be able to accomplish this in one of two ways.
The first is to create the task whenever the "Next Update" field is filled in. You can then keep track of that task via a relationship or with an automation. This might be the simplest solution.
The second, is to use an upcoming feature (scheduled automations) to run a table import once a day. The table import would be setup so that it creates related Tasks for any Documents where the "Next Update" field is equal to "today".
Scheduling for automations is coming in the next few weeks so keep your eyes peeled.
Best,
-Matt
