1

Edited 07/08/13: Apple has an excellent set of WWDC videos that really helped me understand the various date and time classes in Objective-C, and how to correctly perform time calculations/manipulations with them.

"Solutions to Common Date and Time Challenges" (HD video, SD video, slides (PDF)) (WWDC 2013)
"Performing Calendar Calculations" (SD video, slides (PDF)) (WWDC 2011)

Note: links require a free Apple Developer membership.

In this SO question, I asked how I might go about calculating a certain date and time ("next Sunday at 5 PM"). Thanks to the answers I got, I came up with the following code:

 - (NSDate *) toPacificTime
 {
     NSTimeZone *tz = [NSTimeZone timeZoneWithName:@"America/Los_Angeles"];
     NSInteger seconds = [tz secondsFromGMTForDate: self];
     return [NSDate dateWithTimeInterval: seconds sinceDate: self];
 }

 - (void)handleLiveShowReminders
 {
    NSDate *gmtNow = [NSDate date];
    NSDate *now = [gmtNow toPacificTime];

    NSCalendar *calendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
    [calendar setTimeZone:[NSTimeZone timeZoneWithName:@"America/Los_Angeles"]];
    NSDateComponents *dateComponents = [calendar components:NSWeekdayCalendarUnit fromDate:now];
    NSInteger weekday = [dateComponents weekday];

    NSInteger daysTillNextSunday = 8 - weekday;

    int secondsInDay = 86400; // 24 * 60 * 60
    NSDate *nextSunday = [now dateByAddingTimeInterval:secondsInDay * daysTillNextSunday];

    NSDateComponents *components = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit fromDate:nextSunday];
    [components setHour:17];
    [components setMinute:00];
    [components setTimeZone:[NSTimeZone timeZoneWithName:@"America/Los_Angeles"]];
    NSDate *nextSunday5PM = [calendar dateFromComponents:components];

    warningInterval = -300;    // we want the notification to fire 5 minutes beforehand

    NSDate *alertDate = [nextSunday5PM dateByAddingTimeInterval:(NSTimeInterval)warningInterval];

    UILocalNotification* notifyAlarm = [[[UILocalNotification alloc] init] autorelease];
    if (notifyAlarm)
    {
        notifyAlarm.fireDate = alertDate;
        notifyAlarm.timeZone = [NSTimeZone timeZoneWithName:@"America/Los_Angeles"];    
        notifyAlarm.repeatInterval = NSWeekCalendarUnit;
        notifyAlarm.soundName = @"alert.aif";
        notifyAlarm.alertBody = @"LIVE SHOW REMINDER: The live show is about to start!";

        [[UIApplication sharedApplication] scheduleLocalNotification:notifyAlarm];
    }
 }

The problem is that, although this code works for me, it's not working for anyone who's not in the PST timezone, as can be seen by this debug output I received from a user in EST:

number of notifications = 1

Notification #1
===================
Body: LIVE SHOW REMINDER: The live show is about to start!
Details: <UIConcreteLocalNotification: 0x1d5f2200>
{fire date = Sunday, December 9, 2012, 4:30:00 PM Pacific Standard Time,
time zone = America/Los_Angeles (PST) offset -28800,
repeat interval = NSWeekCalendarUnit
repeat count = UILocalNotificationInfiniteRepeatCount,
next fire date = Sunday, December 9, 2012, 4:30:00 PM Eastern Standard Time,
user info = (null)}

What am I doing wrong here?

Donald Burr
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2 Answers2

2

One problem with your logic, just adding 86,400 to your time interval might not be what you want. If for example, the user experiences daylight-savings, then you will be off by an hour, or even a day if it happens near midnight. A better method is to ask the NSCalendar for the result, which takes into account the users local time zone.

NSDate *start = yourDate;
NSDateComponents *oneDay = [NSDateComponents new];
[oneDay setDay:1];

NSCalendar *cal = [NSCalendar currentCalendar];
NSDate *end = [cal dateByAddingComponents:oneDay toDate:start options:0];
Jarson
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0

And NSDate is relative to UTC/GMT. When you "adjust" if for Pacific time you create utter confusion.

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