I'm trying to do some date manipulation with Swift and I'm getting an unexpected result. The webservice will pass in a string date, and then I want to get that month and the previous month. I'm using this code (with input grab and such removed):
import Foundation
let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(identifier: "America/New_York")
formatter.dateFormat = "yyyy-MM-dd"
let date = formatter.date(from: "2018-12-01")!
let prev = Calendar.current.date(byAdding: .month, value: -1, to: date)!
formatter.string(from: date)
formatter.string(from: prev)
So I've got a valid date, and then I subtract a month from it. The first formatted date shows my expected 2018-12-01 but then on the second line, instead of saying 2018-11-01 it says 2018-10-31.
I'm in PST, which is of course 3 hours ahead of EST. If I add 3 hours I'd get the expected strings. However, since both the input and the output strings were done with a formatter using the timezone, why don't I get the expected output?
The problem is that Calendar.current is in a different timezone (for you) than the formatter.
So date is December 1, 2018 at midnight New York time. But that is November 30, 2018 at 9pm local time (PST) for you.
When you subtract one month it is done in local time (Calendar.current) so you get October 30, 2018 at 9pm. Then you format that date to New York time and it results in October 31, 2018 at midnight.
To get the proper results you want a Calendar in the same timezone as the formatter:
var cal = Calendar(identifier: Calendar.current.identifier)
cal.timeZone = formatter.timeZone
let prev = cal.date(byAdding: .month, value: -1, to: date)!
This will give the expected result.
Related
I need to obtain a date from some variable values
So I specify year, month and day and I need a Date as return
Doing the following works except for the day because it return the day input - 1
let todayDate: Date = Calendar.current.startOfDay(for: Date.from(year: 2022, month: 09, day: 05)!)
print("today date = \(todayDate)")
extension Date {
static func from(year: Int, month: Int, day: Int) -> Date? {
let calendar = Calendar(identifier: .gregorian)
var dateComponents = DateComponents()
dateComponents.year = year
dateComponents.month = month
dateComponents.day = day
return calendar.date(from: dateComponents) ?? nil
}
}
And the output is
today date = 2022-09-04 22:00:00 +0000
Date and time can be a bit tricky. The Date struct stores a point in time relative to GMT. If you print it it will show exactly that.
Solution:
Don´t use print, use a proper Dateformatter. To illustrate what I mean use this in a playground:
let date = Calendar.current.startOfDay(for: Date())
print(date)
//2022-09-03 22:00:00 +0000
// when it is 4.th of september 00:00 in my timezone (+- Daylight saving) it is this time in GMT
let formatter = DateFormatter()
formatter.dateFormat = "dd MM yyyy HH:mm:ss"
print(formatter.string(from: date))
//04 09 2022 00:00:00
// this is the time in my timezone
So the issue here is not that it has the wrong time, it is just not presented in the correct time zone.
How to deal with the new year and ISO8601 returning last year as year component.
To my horror, I realized ISO8601DateFormatter was returning 1977 as a year to the 1978-01-01 00:00:00
It took a while to realize this. That turned out is not wrong. Nonetheless, given the specific year of 1978, for the formatted to return 1977 is shocking.
I don't even need the timestamp. How can I reliably retrieve the specified year without having to add a second to every calendar date?
import Foundation
let datestring = "1978/1/1"
var formatter = ISO8601DateFormatter()
formatter.timeZone = TimeZone(abbreviation: "UTC")
formatter.formatOptions = [.withFullDate]
let date2 = formatter.date(from: datestring) ?? Date()
print(date2)
var calendar = Calendar(identifier: .iso8601)
var year = calendar.component(.year, from: date2)
var month = calendar.component(.month, from: date2)
var day = calendar.component(.day, from: date2)
var era = calendar.component(.era, from: date2)
print("year \(year) month \(month) day \(day) era: \(era)")
===
1978-01-01 00:00:00 +0000
year 1977 month 12 day 31 era: 1
By default the Calendar instance will have your local timeZone. You can see this by printing print(calendar.timeZone.abbreviation() ?? "UNKNOWN"). In my case (in Seattle, WA, USA) it prints "PDT". If you simply set your calendar timezone to UTC it prints exactly what you expect:
year 1978 month 1 day 1 era: 1
So, this:
import Foundation
let df = DateFormatter()
df.dateFormat = "yyyy-MM-dd"
df.timeZone = TimeZone(identifier: "Australia/Currie")!
let todayString = (df.string(from: Date()))
print(todayString)
let today = df.date(from: todayString)!
print(today)
Prints:
2021-02-19
2021-02-18 13:00:00 +0000
For reference, today’s date based on my current timezone is the 18th. So it correctly prints the 19th when using an Australian timezone that moves the time ahead.
What I don't understand is why today is a day behind todayString, since it's constructed from the same DateFormatter. Ideally, they should both print the 19th, right?
Basically, what I'm trying to do is have both a date (from a different timezone) and its string representation.
As mentioned in the comments print displays Date instances always in UTC(+0000). For example check
let date = Date()
print(date)
To print the date in your current locale (and time zone) write
print(today.description(with: .current))
I am trying to show today's date as March 26 but it is showing as "March 85" when I use this code.
let formatter = DateFormatter()
formatter.dateFormat = "MMMM DD"
let defaultTimeZoneStr = formatter.string(from: Date())
The problem is that you are using the wrong date format. D is for "day of year". The correct symbol for "day of month" is lowercased d Thats why you are getting 85instead of 26.
Another point you should consider is to set your locale fixed to "en_US_POSIX" if you don't want your date string to reflect the users settings and locale.
Note that you should use Swift native type Date instead of NSDate.
If your intent is to display it respecting the user locale and settings you should use date formatter dateStyle (short, medium, long or full) How do I get the current Date in short format in Swift
If you need a localized date format limited to
month and day only, you can use DateFormatter method dateFormat from template:
class func dateFormat(fromTemplate tmplate: String, options opts: Int, locale: Locale?) -> String?
let df = DateFormatter()
df.dateFormat = DateFormatter.dateFormat(fromTemplate: "MMMMdd", options: 0, locale: .current)
df.string(from: Date()) // "March 27"
I am trying to get a Date object for the next occurring future time where the hour in UTC time is 18. However my code doesn't work as expected. I have the following:
let dateComponents = DateComponents(timeZone: TimeZone(abbreviation: "GMT"), hour: 18)
let date = Calendar.current.nextDate(after: Date(), matching: dateComponents, matchingPolicy: .nextTime)
print(date)
The problem is that this results in 2019-02-09 23:00:00 +0000
The date is for the next occurring time where the hour is 18 in EST.
I would have expected, since the the dateComponents has the timezone set to UTC and the hour to 18, that the date would be 2019-02-09 18:00:00 +0000. Furthermore, changing the timezone seems to have no effect on the nextDate found.
Why doesn't the nextDate function respect the timezone set in the dateComponents passed to it?
It looks like the timezone in DateComponents is ignored.
However when you set the timezone in a new calendar you get correct results.
let dateComponents = DateComponents(hour: 18)
var calendar = Calendar(identifier: .gregorian)
calendar.timeZone = TimeZone(secondsFromGMT: 0)!
let date = calendar.nextDate(after: Date(), matching: dateComponents, matchingPolicy: .nextTime)
print(date) // Optional(2020-09-29 18:00:00 +0000)
I live in GMT time, and running this in a playground produces the expected result you are looking for. You are setting the time zone of your date component, but I would imagine your own calendar (Calendar.current) is set to EST. You would need to account for the offset in EST vs GMT for your required result