[Refactor] JulianDay to be more precise

[mannylopez]

• Refactor static func julianDay(_ date: Date) -> Double to be more precise

static func julianDay(_ date: Date) -> Double {
  return (date.timeIntervalSince1970 * 1000) / (1000 * 60 * 60 * 24) - 0.5 + 2440588
}

[mannylopez]

Preferred to keep more-complicated method, but removed the rounding down to 4 places

Decided to refactor it to since it gets rid of the final fatalError