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Why is dateRemainingText2 giving different result from dateRemainingText?
Obviously dateRemainingText2 is wrong.

Here's my code:

import Foundation let startDate = Calendar.current.date(from: DateComponents(year: 2022, month: 5, day: 1)) ?? .now let endDate = Calendar.current.date(from: DateComponents(year: 2020, month: 6, day: 2)) ?? .now let dateComponentsFormatter = DateComponentsFormatter() dateComponentsFormatter.allowedUnits = [.year, .month, .day] dateComponentsFormatter.unitsStyle = .full var dateRemainingText = dateComponentsFormatter.string(from: startDate, to: endDate)! // -1 year, 10 months, 29 days let dateComponents = Calendar.current.dateComponents([.year, .month, .day], from: startDate, to: endDate) var dateRemainingText2 = dateComponentsFormatter.string(from: dateComponents) // -1 year, 11 months, 1 day
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  • Because you are using two different methods? Commented May 20, 2022 at 17:18
  • But I am using the same startDate and endDate Commented May 20, 2022 at 17:23
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    My explanation is that dateComponents(_,from:to:) calculates the absolute difference of each component. On the other hand string(from:to:) calculates the distance between the two dates. Commented May 20, 2022 at 17:23

1 Answer 1

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DateComponentsFormatter.string(from:to:) and DateComponentsFormatter.string(from:) are different methods and so they can do different things.

From some experimentation, we can see that string(from:) outputs a string that describes the sum of all the date components in the DateComponents passed in.

This just happens to be the same as the output for string(from:to:) for most of the cases where the components are all positive.

Examples:

// 1 month + 7 days var dateComponents = DateComponents() dateComponents.month = 1 dateComponents.day = 7 // 1 month, 7 days print(dateComponentsFormatter.string(from: dateComponents)!) // 7 days - 1 month dateComponents.month = -1 dateComponents.day = 7 // -24 days print(dateComponentsFormatter.string(from: dateComponents)!) // 1 month - 7 days dateComponents.month = 1 dateComponents.day = -7 // 24 days print(dateComponentsFormatter.string(from: dateComponents)!) // 1 month + 30 days // note that adding 1 month, it's February, and there are only 28 days dateComponents.month = 1 dateComponents.day = 30 // 2 months, 2 days print(dateComponentsFormatter.string(from: dateComponents)!) // -10 months - 30 days // note that after subtracting 10 months, it is March of the previous year, // which also happens to be a leap year, so February has 29 days // Subtracting 30 days from that will bring us to January, with one day left // which is, where the extra month and day came from dateComponents.month = -10 dateComponents.day = -30 // -11 months, 1 day print(dateComponentsFormatter.string(from: dateComponents)!) // -1 year - 10 months - 30 days // similar to above, except not a leap year // so we have 2 days left after subtracting from a 28-day February. dateComponents.year = -1 dateComponents.month = -10 dateComponents.day = -30 // -1 year, 11 months, 2 days print(dateComponentsFormatter.string(from: dateComponents)!)

The DateComponents used in that last case is what Calendar.dateComponents(_:from:to:) in your code.

On the other hand, string(from:to) is the designated method to format the period between two dates.

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