public UnionStrategy getUnionStrategy() {
        return unionStrategy;
    }
Sets the assumed timestampFormat trait for timestamps with no
timestampFormat trait. The provided value is expected to be a string.

<p>Defaults to "date-time" if not set. Can be set to "date-time",
"epoch-seconds", or "http-date".

@param defaultTimestampFormat The default timestamp format to use when none is set.
smithy-jsonschema/src/main/java/software/amazon/smithy/jsonschema/JsonSchemaConfig.java