Having trouble trying to subtract a number from a date/time and setting that value in a textField

Question

I am working on an app that has a startTime, endTime and duration. The user can set the endTime by click a button and it sets the value to "now" in the format 12:02:03 PM. I then want to be able to enter a duration time in minutes, let's say 20 minutes.

I have everything working where I can read the duration in real time as well as see the current time. The issue is when I try to create a function to subtract the duration from the endTime. I cannot seem to get the syntax or the formatting correct.

I've done quite a bit of searching for examples of this. Here is what I came across so far.

How to add minutes to current time in swift

How to subtract date components?

How to get the current time as datetime

func controlTextDidChange(_ obj: Notification) {
    let enteredValue = obj.object as! NSTextField
    timeString(time: enteredValue.doubleValue)
}

func timeString(time: TimeInterval) {
    formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
    let myString = formatter.string(from: Date())
    let yourDate = formatter.date(from: myString)
    formatter.dateFormat = "hh:mm:ss a"
    yourDate!.addingTimeInterval(0-time)
    let wtf = formatter.string(from: yourDate!)
    startTime.stringValue = wtf
}

The controlTextDidChange function is watching the durationTextField and I am able to print to console the input. I then want to be able to run the timeString function with the durationTextField value and subtract it from the endTime and then set that value to startTime.

One thing that is weird is Xcode tells me:

Result of call to 'addingTimeInterval' is unused

score 0 · Answer 1 · answered Aug 01 '19 at 17:38

It's not weird, the warning tells you that addingTimeInterval creates and returns a new date.

Just do use the Result of call to 'addingTimeInterval'

The conversion Date to String and back to Date is pointless.

func timeString(time: TimeInterval) {
    formatter.dateFormat = "hh:mm:ss a"
    let newDate = Date().addingTimeInterval(-time)
    let wtf = formatter.string(from:newDate)
    startTime.stringValue = wtf
}

score 0 · Accepted Answer · answered Aug 01 '19 at 17:38

You are taking too many steps. Just create a Date that is time seconds from "now". Then convert that Date to a String.

func timeString(time: TimeInterval) {
    let startDate = Date(timeIntervalSinceNow: -time)
    formatter.dateFormat = "hh:mm:ss a"
    let wtf = formatter.string(from: startDate)
    startTime.stringValue = wtf
}

I'm assuming you want startDate to be time seconds before now.

score 0 · Answer 3 · answered Aug 04 '19 at 05:33

With the help of vadian and rmaddy I was able to make it work.

Here is my working code

func timeString(time: TimeInterval) {
    formatter.dateFormat = "hh:mm a"
    let endTimeValue = formatter.date(from: endTime.stringValue)
    let newTime = endTimeValue!.addingTimeInterval(-time * 60)
    let newtimeString = formatter.string(from:newTime)
    startTime.stringValue = newtimeString
}

Having trouble trying to subtract a number from a date/time and setting that value in a textField

3 Answers3