Unix Timestamp in Swift

Use `Date` and `ISO8601DateFormatter` from Foundation.

let ts = Int(Date().timeIntervalSince1970)
// 1706745600

let date = Date(timeIntervalSince1970: 1706745600)
// 2024-01-31 22:40:00 +0000

let formatter = ISO8601DateFormatter()
formatter.string(from: Date())
// '2024-01-31T22:40:00Z'

let formatter = ISO8601DateFormatter()
let date = formatter.date(from: "2024-01-31T22:40:00Z")