NSDate Format outputting wrong date

问题:

I have a NSString (ex. \"2011-04-12 19:23:39\"), and what I did to format it to a NSDate was the following:

[inputFormatter setDateFormat:@\"yyyy-MM-dd HH:mm:ss\"];

NSDate *date = [inputFormatter dateFromString:newDateString];

but what it outputs when I nslog the date is this:

2011-04-12 23:23:39 +0000

which is about 4 hours off. Is there something I missed? Possibly a time zone problem?

回答1:

NSDateFormatter use the current device timezone when it created the NSDate object. NSDate stores the date/time in GMT. Therefore by default NSLog will output the date/time in GMT+0. So, there\'s nothing wrong with your code. Now if you want to output the NSDate to your current timezone, your will have to use a NSDateFormatter object.

回答2:

The answer in short, is the Date is being returned GMT unless specified otherwise. You can set your timezone to get the correct date. If you plan on using the date in the app to set anything ( like localNotification time or Event ) you will need to do something special with the date because if you set the date in the iPhone it will be set as GMT time and will be off by a few hours. ( in your case 4 hours ). I do this exact thing I just described in one of my apps.

I made a mess of trying to get this to work correctly without having the hours be off. It was a huge PITA to figure out but its working now. I have copied, pasted, and edited my code to share. Again, its messy but it works! The pickerChanged is getting its info from a UIDatePicker

Using the code below. To answer your question, you can stop at \"destinationDate\". That will return to you the corrected time for your current time zone. I just provided the extra incase you were trying to use the date in the Phone somewhere.

NOTE: for a quick example i put the Event reminder in the same function as the datepicker, you will NOT want to do that otherwise you will have alot of reminders set everytime the wheel scrolls in the datepicker.

The code is below.

 - (void)pickerChanged:(id)sender
    {

        NSLog(@\"value: %@\",[sender date]);

        NSDate* date= [sender date]; 
        NSDateFormatter *formatter=[[[NSDateFormatter alloc]init]autorelease]; 
        [formatter setDateFormat:@\"MM/dd/yyyy hh:mm:ss a\"];
        [formatter setTimeZone:[NSTimeZone systemTimeZone]];

        [formatter setTimeStyle:NSDateFormatterLongStyle]; 

        NSString *dateSelected =[formatter stringFromDate:date]; 

        NSString *timeZone = [dateSelected substringFromIndex:12];



    NSTimeZone* destinationTimeZone = [NSTimeZone systemTimeZone];

    //here we have to get the time difference between GMT and the current users Date (its in seconds)
        NSInteger destinationGMTOffset = [destinationTimeZone secondsFromGMTForDate:date];

    //need to reverse offset so its correct when we put it in the calendar
        correctedTimeForCalendarEvent = destinationGMTOffset + (2*(-1*destinationGMTOffset));

    //date to enter into calendar (we will use the correctedTimeForCalendarEvent to correct the time otherwise it will be off by a few hours )
       NSDate * destinationDate = [[[NSDate alloc] initWithTimeInterval:destinationGMTOffset sinceDate:date] autorelease];
        NSDate * dateForReminder = destinationDate;
        // return destinationDate;
        NSLog(@\"value: %@ - %@\",destinationDate,dateForReminder);

//DO NOT put this code in this same function this is for a quick example only on StackOverflow
//otherwise you will have reminders set everytime the users scrolled to a different time
    //set event reminder
    //make sure to import EventKit framework

        EKEventStore *eventDB = [[[EKEventStore alloc] init]autorelease];
        EKEvent *myEvent  = [EKEvent eventWithEventStore:eventDB];
        NSString * eventTitle = [NSString stringWithFormat:@\"%@ - %@\",app.dealerBusinessName,serviceOrComments.text];
        myEvent.title = eventTitle;

    //double check date one more time
        NSLog(@\"value: %@\",destinationDate);

    //set event time frame (1 hour) the \"initWithTimeInterval\" is where we account for the users timezone by adding the correctedTime from GMT to the calendar time ( so its not off by hours when entering into calendar)
        myEvent.startDate = [[[NSDate alloc] initWithTimeInterval:correctedTimeForCalendarEvent sinceDate:destinationDate ]autorelease];
        myEvent.endDate   = [[[NSDate alloc] initWithTimeInterval:3600 sinceDate:myEvent.startDate]autorelease];
        myEvent.allDay = NO;

    //set event reminders 1 day and 1 hour before
        myAlarmsArray = [[[NSMutableArray alloc] init] autorelease];
        EKAlarm *alarm1 = [EKAlarm alarmWithRelativeOffset:-3600]; // 1 Hour
        EKAlarm *alarm2 = [EKAlarm alarmWithRelativeOffset:-86400]; // 1 Day
        [myAlarmsArray addObject:alarm1];
        [myAlarmsArray addObject:alarm2];
        myEvent.alarms = myAlarmsArray;



        [myEvent setCalendar:[eventDB defaultCalendarForNewEvents]];

        NSError *err;

        [eventDB saveEvent:myEvent span:EKSpanThisEvent error:&err]; 

        if (err == noErr) {
            //no error, but do not show alert because we do that below.
        }

    }

回答3:

Your data and date formatter omit the TimeZone specifier. So something like this:

[inputFormatter setDateFormat:@\"yyyy-MM-dd HH:mm:ssZ\"];

Would work - the Z is the timezone specifier and will parse both numeric offsets and timezone codes. Although in your case as your input date has no TimeZone information it won\'t work.

Your correct Time string should be like \"2011-04-12 19:23:39 -0400\" or \"2011-04-12 19:23:39 EST \"

Depending on where you get your date from, you should fix that to produce a fully qualified date if you can\'t do that, you will have to agree timezone offsets with the server or simply \'hard code\' a timezone offset and add that number of seconds to your NSDate.

回答4:

The date is being logged as a UTC date can be seen by the +0000 at the end. The date format you are using to parse the string assumes your local time zone which is presumably 4 hours behind UTC with daylight savings and the standard -5 hours.

回答5:

Use -[NSDateFormatter setTimeZone:] to provide the date formatter with timezone information. You can use the local time zone, or if you have a fixed time zone associated with the date information, I recommend creating the timezone with the name (such as \"America/East\") rather than the abbreviation (such as \"EST\" or \"EDT\"), since the name does not force daylight savings into effect, but uses the correct daylight savings offset for that date in that timezone.