Workflow - Create a task based on a custom property with a date picker

I'm trying to create a workflow that is triggered by a property that I have created for our organization that has a date picker field. There doesn't seem to be a way to do this currently. 

 

For example:

Custom property X has a date picker where a future date as been selected. I would like to create a task that would be created 7 days prior to the date chosen. 

 

There could be many more applications for this but this is one example. 

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