Derivative contracts, either from traditional finance or reproduced in the crypto world in the form of smart contracts, are a very popular financial instrument widely used in the industry and of great interest to supervisors, financial intermediaries, and investment banks. With the expansion of Decentralized Finance (DeFi) environments, where no or little intermediation is involved, these contracts are also being deployed into a technological infrastructure, the blockchain, in the form of executable scripts. Unfortunately, these scripts are often criticized as their business logic is very complex, also due to the need to handle the temporal dimension, the implementations are hard to explain and communicate, and the adopted languages are highly procedural and very technical. Consequently, DeFi derivatives are often untrusted by non-IT users and hard to supervise. This joint work of the researchers of the Central Bank of Italy and the Polytechnic University of Milan aims at bridging this gap by presenting the first declarative, logic-based, and executable implementation of a DeFi derivative contract. In particular, leveraging the extensive experience of the database and reasoning-based AI communities about logical languages for knowledge representation and reasoning, we adopt the DatalogMTL language for temporal reasoning to encode the Ethereum Perpetual Future (ETH-PERP) smart contract. We show how the language conveys simplicity, understandability, and transparency for non-technical users. To validate our implementation, we execute the smart contract in Vadalog, a modern reasoner supporting DatalogMTL.