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You can find the documentation on the suitable values and other uses for format_datetime here.
You can find the definition of the results_tc method here and any additional information here.
You must properly define the last_run input in your application.
You should not make the last_run parameter required in any context. The initial run of the application should have this blank so that a default value is used.
You should not show the last_run parameter in the Feed Deployer Wizard (if used).