let from = Variable<Date?>(nil)
let to = Variable<Date?>(nil)

let isOnVariable = Variable<Bool>(false)
var isOn: Observable<Bool> {
    return Observable.combineLatest(from.asObservable(), to.asObservable()) { [weak self] to, from in
        switch (from, to) {
        case (.none, .none):
            self?.isOnVariable.value = false
            return false
        default:
            self?.isOnVariable.value = true
            return true
        }
    }
}