* discount expiration validation and testing
* integration testing
* double quotes
* add iso8601 package
* api testing
* add joi validation of start and end dates as well as valid_duration
* valid_duration column
* service testing
* discount validation in services
* integration test with invalid date interval
* include valid_duration when fetching a discount
* rename variable for clarity
* add test for dynamic discount with expiration date
* remove debug code
* adjust tests to reflect valid_duration being included in default fields
* additional discount update validation
* fixed failing test
* set ends_at on dynamic discount creation
* discount integration tests
* removed unused console.log
* removed validation of dynamic discounts by duration and added ends_at to dynamic discount creation
* integration tests for dynamic discount with and without duration
* optional valid duration for dynamic discounts
* allow nullable dynamic discount durations
* expect assertions
* fix unit test after change to dynamic discounts without duration
* change to date instead of string
* add assertions
* error handling
* addressed feedback
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