Illustrating the differences between encoding through a encoding container (recommended) vs. calling encode(to:) directly on the encoder (generally not recommended).

CodableContainerVsEncoder.swift

import Foundation

struct ContainerEncoded: Encodable {
    var date: Date = .now

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(date)
    }
}

struct DirectEncoded: Encodable {
    var date: Date = .now

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

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601
let containerEncoded = try encoder.encode([ContainerEncoded()])
let directEncoded = try encoder.encode([DirectEncoded()])
String(decoding: containerEncoded, as: UTF8.self) // ["2021-11-18T10:47:54Z"]
String(decoding: directEncoded, as: UTF8.self) // [658925237.50108004]