Swift hour and minute from a string to a Date - swift

Question

I'm trying to apply some calcul between two hours.
I receive from firebase my too hours from a String format (HH:mm) and I try to convert it to a Date format (HH:mm). Then apply the calcul before converting again to a string and seed it to the archive in Firebase.
My code:
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "HH:mm"
ref.child("Experience").child(userID!).observeSingleEvent(of: .value) {(snapshot) in
if let value = snapshot.value as? [String:String]{
let value = snapshot.value as? NSDictionary
let total_time = value?["TOTAL_TIME"] as? String ?? "00:00" // Received from Firebase
self.oldDateTotalTime = dateFormatter.date(from: total_time)! //trying to convert into a Date format
print(self.oldDateTotalTime)
self.oldStringTotalTime = total_time //just for test
}
This code didn't work because, I think, of the dateFormatter.
S my questions are:
How can I get my Date from my String?
How can I get after my String from my Date?
edit:
to precise my question, my objective is to subtract one time value to a total time value. I've got a total user experience i.e 700 H 50 min with is given on Firebase like this : 700:50. In an other part I've got some specific experiences i.e 1h 40min with is given as before 1:40 in firebase.
My request is to subtract the specific experience from the global.
700:50 - 1:40 = 699:10
On my current Swift experience I only use Date() & DateFormatter to deal with the time and I don't know how to deal only with time and not with date ...
Hope you will better understand this with my edit

Answer 1

If you need, here is the regex code ; can certainly be optimised.
func stringToTime(_ timeStr: String) -> (Int, Int) {
var hours = 0
var minutes = 0
let patternH = "[0-9]*[:]" // digits, followed by :
let regexH = try! NSRegularExpression(pattern: patternH, options: .caseInsensitive)
if let matchH = regexH.firstMatch(in: timeStr, range: NSRange(0..<timeStr.utf16.count)) {
let hStr = String(timeStr[Range(matchH.range(at: 0), in: timeStr)!]).dropLast()
hours = Int(hStr) ?? 0
let patternM = "[:][0-9]{1,2}" // 1 or 2 digits
let regexM = try! NSRegularExpression(pattern: patternM, options: .caseInsensitive)
if let matchM = regexM.firstMatch(in: timeStr, range: NSRange(0..<timeStr.utf16.count)) {
let mStr = String(timeStr[Range(matchM.range(at: 0), in: timeStr)!]).dropFirst()
minutes = Int(mStr) ?? 0
}
}
return (hours, minutes)
}
let timeStr = "700:50"
let time1 = stringToTime("700:50")
let time2 = stringToTime("1:40")
var time = (time1.0 - time2.0, time1.1 - time2.1)
if time.1 < 0 {
time.1 = time.1 + 60
time.0 -= 1
}
print(time)