Calculating the difference between two dates in Swift

Question

I've seen many approaches how to compute the difference between two dates in terms of a particular date component, eg in days, hours, months etc. (see this answer on Stackoverflow ):

Calendar.current.dateComponents([.hour], from: fromDate, to: toDate).hour
Calendar.current.dateComponents([.day], from: fromDate, to: toDate).day
Calendar.current.dateComponents([.month], from: fromDate, to: toDate).month

What I haven't seen is how to make calculations with the actual Date objects. Something like

func computeNewDate(from fromDate: Date, to toDate: Date) -> Date    
     let delta = toDate - fromDate
     let today = Date()
     if delta < 0 {
         return today
     } else {
         return today + delta
     }
}

I have seen the DateInterval type which as introduced in iOS 10 but according to the documentation

[it] does not support reverse intervals ie intervals where the duration is less than 0 and the end date occurs earlier in time than the start date.

That makes it inherently difficult to calculate with dates – especially when you don't know which one is the earlier date.

Is there any clean and neat approach to compute time differences between Date s directly (and adding them to Date instances again) without computing with their timeIntervalSinceReferenceDate ?

Answer 1

I ended up creating a custom operator for Date :

extension Date {

    static func - (lhs: Date, rhs: Date) -> TimeInterval {
        return lhs.timeIntervalSinceReferenceDate - rhs.timeIntervalSinceReferenceDate
    }

}

With this operator I can now compute the difference between two dates on a more abstract level without caring about timeIntervalSinceReferenceDate or what exactly the reference date is – and without losing precision, for example:

let delta = toDate - fromDate

Obviously, I didn't change much, but for me it's a lot more readable and consequent: Swift has the + operator already implemented for a Date and a TimeInterval :

 /// Returns a `Date` with a specified amount of time added to it. public static func + (lhs: Date, rhs: TimeInterval) -> Date

So it's already supporting

Date + TimeInterval = Date

Consequently, it should also support

Date - Date = TimeInterval

in my opinion and that's what I added with the simple implementation of the - operator. Now I can simply write the example function exactly as mentioned in my question:

func computeNewDate(from fromDate: Date, to toDate: Date) -> Date    
     let delta = toDate - fromDate // `Date` - `Date` = `TimeInterval`
     let today = Date()
     if delta < 0 {
         return today
     } else {
         return today + delta // `Date` + `TimeInterval` = `Date`
     }
}

It might very well be that this has some downsides that I'm not aware of at this moment and I'd love to hear your thoughts on this.

Answer 2

You can extension with custom operator, and return tuples

extension Date {

    static func -(recent: Date, previous: Date) -> (month: Int?, day: Int?, hour: Int?, minute: Int?, second: Int?) {
        let day = Calendar.current.dateComponents([.day], from: previous, to: recent).day
        let month = Calendar.current.dateComponents([.month], from: previous, to: recent).month
        let hour = Calendar.current.dateComponents([.hour], from: previous, to: recent).hour
        let minute = Calendar.current.dateComponents([.minute], from: previous, to: recent).minute
        let second = Calendar.current.dateComponents([.second], from: previous, to: recent).second

        return (month: month, day: day, hour: hour, minute: minute, second: second)
    }

}

Using:

let interval = Date() - updatedDate
print(interval.day)
print(interval.month)
print(interval.hour)

Answer 3

Simply toDate.timeIntervalSince(fromDate) .

To reimplement your function without adding any extension:

func computeNewDate(from fromDate: Date, to toDate: Date) -> Date  {  
     let delta = toDate.timeIntervalSince(fromDate) 
     let today = Date()
     if delta < 0 {
         return today
     } else {
         return today.addingTimeInterval(delta) 
     }
}

Answer 4

我找到了一个内置解决方案来计算两个日期之间的差异。

let delta = toDate.timeIntervalSince(fromDate)

Answer 5

您可以使用 ：

let delta = fromDate.distance(to: toDate)

Answer 6

How about something like…

func computeNewDate(from fromDate: Date, to toDate: Date) -> Date {
    let delta = Calendar.current.dateComponents([.second], from: fromDate, to: toDate).second!
    return Calendar.current.date(byAdding: .second, value: delta, to: Date())!
}