protocol Calculable {
  // The calc logic you were going to have in SessionHelper
  func calc()
}

protocol Timeable {
  // The same timeElapsed property you had before
  var timeElapsed: TimeInterval { get }
}

class Session {
  var start: Date
  var end: Date

  init() {
    self.start = Date()
    self.end = Date()
  }
}

// Mark: - Timeable Conformance -
extension Session: Timeable {
  var timeElapsed: TimeInterval {
        return calcSomething(start, end, pauses, resumes)
    }
}

// MARK: - Calculable Conformance
extension Session: Calculable {
  func calc() {
    // Perform the logic for calculating right in here using timeElapsed computed property
  }
}