timeIntervalSince1970 is a floating point count of the number of seconds since the UNIX epoch. The numbers after the decimal point aren’t milliseconds per se, but fractions of a second. You can get milliseconds by rounding the value to three decimal places, microseconds by rounding to 6, and so on.
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Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"