I've seen many approaches how to compute the difference between two dates in terms of a particular date component, e.g. 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 i.e. 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
?
Date Difference Extension in Swiftlet formatter = DateFormatter() formatter. dateFormat = "yyyy/MM/dd HH:mm" let xmas = formatter. date(from: "2021/12/24 00:00") let newYear = formatter. date(from: "2022/01/01 00:00") print(newYear!
To subtract hours from a date in swift we need to create a date first. Once that date is created we have to subtract hours from that, though swift does not provide a way to subtract date or time, but it provides us a way to add date or date component in negative value.
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.
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