Why do my simple date calculations sometimes fail in Swift 3.1?
I have a unit test that looks like this:
func testManyYearsAgo() {
for year in 2...77 {
let earlierTime = calendar.date(byAdding: .year, value: 0 - year, to: now)
// print(year)
// print(dateDifference.itWasEstimate(baseDate: now, earlierDate: earlierTime!))
XCTAssertEqual(dateDifference.itWasEstimate(baseDate: now, earlierDate: earlierTime!), "\(year) years ago")
}
}
now
defined above as soon as Date()
calendar
Calendar.current
He is testing a class that looks something like this:
class DateDifference {
func itWasEstimate(baseDate: Date, earlierDate: Date) -> String {
let calendar = Calendar.current
let requestedComponent: Set<Calendar.Component> = [ .year, .month, .day, .hour, .minute, .second]
let timeDifference = calendar.dateComponents(requestedComponent, from: baseDate, to: earlierDate)
if timeDifference.year! < 0 {
if timeDifference.year! == -1 {
return "Last year"
} else {
return "\(abs(timeDifference.year!)) years ago"
}
}
return ""
}
}
When I run a unit test, I usually (but not always) get an error like:
XCTAssertEqual failed: ("30 years ago") is not equal to ("31 years ago")
These errors usually start after the year value exceeds 12.
If I uncomment the print instructions it works fine no matter how many times I run the code.
This leads me to believe that there might be some strange asynchronous thing going on there, but I can't tell by looking. I'm relatively new to rapid development, so there might be something fundamental I'm missing.
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Here's a self-contained reproducible example demonstrating the Problem:
var calendar = Calendar(identifier: .gregorian)
calendar.locale = Locale(identifier: "en_US_POSIX")
calendar.timeZone = TimeZone(secondsFromGMT: 0)!
let formatter = DateFormatter()
formatter.calendar = calendar
formatter.dateFormat = "yyyy-MM-dd HH:mm:ss.SSS"
let d1 = DateComponents(calendar: calendar, year: 2017, month: 1, day: 1, hour: 0,
minute: 0, second: 0, nanosecond: 456 * Int(NSEC_PER_MSEC)).date!
print("d1:", formatter.string(from: d1))
let d2 = calendar.date(byAdding: .year, value: -20, to: d1)!
print("d2:", formatter.string(from: d2))
let comps: Set<Calendar.Component> = [ .year, .month, .day, .hour, .minute, .second, .nanosecond]
let diff = calendar.dateComponents(comps, from: d1, to: d2)
print(diff)
print("difference in years:", diff.year!)
Output
d1: 2017-01-01 01: 00: 00.456 d2: 1997-01-01 01: 00: 00.456 year: -19 month: -11 day: -30 hour: -23 minute: -59 second: -59 nanosecond: -999999756 isLeapMonth: false difference in years: -19
Due to rounding errors ( Date
uses a binary floating point number as internal representation), the difference is calculated as a tiny bit less than 20 years old, and the year difference looks like -19 instead of the expected -20.
As a workaround, you can round the dates to full seconds, which seems to fix the problem:
let baseDate = Date(timeIntervalSinceReferenceDate: baseDate
.timeIntervalSinceReferenceDate.rounded())
let earlierDate = Date(timeIntervalSinceReferenceDate: earlierDate
.timeIntervalSinceReferenceDate.rounded())
You may also want to consider posting a bug report to Apple.
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I debugged a bit and found that sometimes it timeDifference
goes offline for 1 day.
What I did, I put this line after initialization timeDifference
:
print("\(timeDifference.year!) \(timeDifference.month!) \(timeDifference.day!)")
The expected output was something like this
-2 0 0
-3 0 0
-4 0 0
-5 0 0
-6 0 0
-7 0 0
...
However, the actual output contains something like:
-38 0 0
-38 -11 -30
-39 -11 -30
-40 -11 -30
...
-55 -11 -30
-57 0 0
Apparently, after a few years month
, day
they become -11
and -30
accordingly.
How do you fix this?
Unfortunately, I cannot find the root cause of this problem. However, I came up with a brute force solution:
if timeDifference.year! < 0 {
if timeDifference.year! == -1 {
return "Last year"
} else {
if timeDifference.month == -11 && timeDifference.day == -30 {
return "\(abs(timeDifference.year!) + 1) years ago"
} else {
return "\(abs(timeDifference.year!)) years ago"
}
}
}
I check if the time difference is disabled by 1 day. If so, add 1 to abs year
.
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