swift - Convert a String to a Date and then to a String in a different format

问题

Suppose I have a simple String, extracted from an array of strings (representing dates) -

var myDateString = "2015-11-25 04:31:32.0"

Now, I want to convert it to a string that looks like Nov 25, 2015.

For that, I assume I need to -

get a NSDate date object from string using NSDateFormatter
get a string from that NSDate object by using some functionality (which I've been unable to find)

I've been trying this in Playground, but have been unable to solve such a simple problem.

Here is what I have tried -

Line 1| var dateFormatter = NSDateFormatter()
Line 2| dateFormatter.locale = NSLocale.currentLocale()
Line 3| dateFormatter.dateFormat = "YYYY-MM-dd HH:mm:ss.A"
Line 4| let somedate = dateFormatter.dateFromString(str)
Line 5| let somedateString = dateFormatter.stringFromDate(somedate!)

In the sidebar of Xcode, line 4 prints "Nov 25, 2015, 12:00 AM", but when I try to print a string from somedate, I get "2015-11-25 00:00:00.0".

I haven't found a single proper explanation of NSDates anywhere.

This was so much easier in Java with parse() and format() functions for Dates.

回答1:

Updated for Swift 3.0

You need two date formatters - one to make sense of your input string and convert to a NSDate, and a different formatter to create the output string

    let myDateString = "2016-01-01 04:31:32.0"

    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss.A"
    let myDate = dateFormatter.date(from: myDateString)!

    dateFormatter.dateFormat = "MMM dd, YYYY"
    let somedateString = dateFormatter.string(from: myDate)

I have updated this answer to change the date formatter from YYYY to yyyy. In most cases, there will be no difference between the two, but YYYY may treat the first few days of the year as being part of the last complete week of the previous year.

The Apple developer guides explain it like this.

A common mistake is to use YYYY. yyyy specifies the calendar year whereas YYYY specifies the year (of “Week of Year”), used in the ISO year-week calendar. In most cases, yyyy and YYYY yield the same number, however they may be different. Typically you should use the calendar year.

回答2:

You will need two date formatters as others suggested. Additionally, one of your issues is that you are using .A instead of .S for the fractional seconds. (See the link to Unicode Date Format Specification as pointed out in @MartinR's comment)

.A is Milliseconds in day

.S is Fractional Second

The the following in a Playground:

let dateString = "2015-11-25 04:01:32.0"

let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss.S"                 // Note: S is fractional second
let dateFromString = dateFormatter.dateFromString(dateString)      // "Nov 25, 2015, 4:31 AM" as NSDate

let dateFormatter2 = NSDateFormatter()
dateFormatter2.dateFormat = "MMM d, yyyy"

let stringFromDate = dateFormatter2.stringFromDate(dateFromString!) // "Nov 25, 2015" as String

来源：https://stackoverflow.com/questions/34826282/swift-convert-a-string-to-a-date-and-then-to-a-string-in-a-different-format

标签

ios

string

swift

nsdate