What should be the Coercing input & output types for GraphQL Java custom Scalars?

GraphQL Java Scalars: https://github.com/graphql-java/graphql-java/blob/14.x/src/main/java/graphql/Scalars.java#L204

In all these cases the Coercing Input and Output types are the same. For instance: Coercing<Boolean, Boolean>

Say, in-case I wanted to create a custom Scalar for DateTime. Should I implement it with Coercing<DateTime, DateTime> OR Coercing<DateTime, String>?

Any insight on this will be helpful.