You can retrieve promotions that are active at a particular time. Implement this use case in your storefront by following the steps below:
Set Shopper Context
Follow the instructions in Set and Retrieve Shopper Context.
Set Effective Date
To retrieve promotions that are active within a particular time window, set an effective date as a context parameter.

{"effectiveDateTime": "2020-12-20T00:00:00Z"
}
note: If the effect date time isn’t provided, the current dateTime is assumed.