なるほど。 @swift-5.0.3
import Foundation let iso8601WithFractionalSecondFormatter: DateFormatter = { let formatter = DateFormatter() formatter.locale = Locale(identifier: "en_US_POSIX") formatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSSSSS" formatter.timeZone = TimeZone(secondsFromGMT: 0) return formatter }() let gregorian = Calendar(identifier: .gregorian) let utc = TimeZone(identifier: "UTC")! let components = gregorian.dateComponents(in: utc, from: Date()) func date(from components: DateComponents, with nanosecond: Int) -> Date { var components = components components.nanosecond = nanosecond return components.date! } let date1 = date(from: components, with: 999499976) print(iso8601WithFractionalSecondFormatter.string(from: date1)) print(gregorian.dateComponents(in: utc, from: date1).nanosecond!) let date2 = date(from: components, with: 999499977) print(iso8601WithFractionalSecondFormatter.string(from: date2)) print(gregorian.dateComponents(in: utc, from: date2).nanosecond!)