Binary operator '-' cannot be applied to two (NSDate?) operators - sum

Question

Trying to get a total amount of time from a start time and end time.
I have the DateFormatter correct as in Extension.swift below but I am getting confused on the way to calculate bottomtime - starttime.
Any help is appreciated. Thanks
Extension.swift
extension NSDate{
var bottomtimestringValue: String{
return self.toString()
}
func tobottomtimeString() -> String {
let formatter = NSDateFormatter()
formatter.dateFormat = "yyyy-MMM-dd"
let str = formatter.stringFromDate(self)
return str
}
}
extension String{
var bottomtimedateValue: NSDate?{
return self.toDate()
}
func tobottomtimeDate() -> NSDate? {
let formatter = NSDateFormatter()
formatter.dateFormat = "yyyy-MMM-dd"
if let date = formatter.dateFromString(self) {
return date
}else{
// if format failed, Put some code here
return nil // an example
}
}
}
Adddivelogviewcontroller.swift
var a = (textFieldStartTime.text.starttimedateValue)
var b = (textFieldEndTime.text.endtimedateValue)
var sum = b - a
textFieldBottomTime.text = "\(sum)"

Answer 1

That's correct-- the - operator is not defined for NSDate?, so you can't find the difference that way. Since a and b are both NSDate?, you could find the difference like this:
if let dateA = a, dateB = b {
let difference = dateA.timeIntervalSinceReferenceDate - dateB.timeIntervalSinceReferenceDate
}
Here, difference's type will be NSTimeInterval?.
Or if you prefer, you could add a definition of - for NSDate?, which might look like this:
func -(lhs:NSDate?, rhs:NSDate?) -> NSTimeInterval? {
if let left=lhs, right=rhs {
return left.timeIntervalSinceReferenceDate - right.timeIntervalSinceReferenceDate
} else {
return nil
}
}
Add that and you can use - above.

Answer 2

OK, not really sure if thats working or not but i get no errors. What I need this to do calculate after the end time picker OK botton is pressed
func OK3ButtonTapped(sender: UIBarButtonItem) {
self.textFieldEndTime.endEditing(true)
self.textFieldEndTime.text = endtimePickerView.date.endtimestringValue
var a = (textFieldStartTime.text.starttimedateValue)
var b = (textFieldEndTime.text.endtimedateValue)
var difference: String = ""
if let dateA = a, dateB = b {
let difference = dateA.timeIntervalSinceReferenceDate - dateB.timeIntervalSinceReferenceDate
}
textFieldBottomTime.text = "\(difference)"
}