AnyEncodable.swift

import Foundation

struct AnyEncodable: Encodable {
    let value: Encodable
    init(_ value: Encodable) {
        self.value = value
    }

    func encode(to encoder: Encoder) throws {
        try value.encode(to: encoder)
    }
}

struct Model<T : Encodable> : Encodable {
    let value: T
    init(_ value: T) {
        self.value = value
    }
}

let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .formatted(formatter)

let now = Date()
let d1 = try encoder.encode(Model(now))
let d2 = try encoder.encode(Model(AnyEncodable(now)))

print(String(data: d1, encoding: .utf8)!)
print(String(data: d2, encoding: .utf8)!)