No, it only uses an int if the value is less than 0xffffffff, otherwise it uses a double. That means it's ok up to 2^53 milliseconds, at which point it starts

No, it only uses an int if the value is less than 0xffffffff, otherwise it uses a double. That means it's ok up to 2^53 milliseconds, at which point it starts losing precision (you can't represent every integer above 2^53 as a double, it gets rounded). It doesn't really matter because no computer that exists today will still be running in 2^53 milliseconds.