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PO(iOS, Objective-C) Handling different time zones when calculating dates/times
 

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    <p><strong>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.</strong></p> <p><strong>"Solutions to Common Date and Time Challenges" (<a href="http://devstreaming.apple.com/videos/wwdc/2013/227xax5xif2s7s531dsmfs1afo2/227/227-HD.mov?dl=1" rel="nofollow">HD video</a>, <a href="http://devstreaming.apple.com/videos/wwdc/2013/227xax5xif2s7s531dsmfs1afo2/227/227-SD.mov?dl=1" rel="nofollow">SD video</a>, <a href="http://devstreaming.apple.com/videos/wwdc/2013/227xax5xif2s7s531dsmfs1afo2/227/227.pdf?dl=1" rel="nofollow">slides (PDF)</a>) (WWDC 2013)</strong><br> <strong>"Performing Calendar Calculations" (<a href="https://developer.apple.com/videos/wwdc/2011/includes/performing-calendar-calculations.html#performing-calendar-calculations" rel="nofollow">SD video</a>, <a href="https://developer.apple.com/devcenter/download.action?path=/wwdc_2011/adc_on_itunes__wwdc11_sessions__pdf/117performing_calendar_calculations.pdf" rel="nofollow">slides (PDF)</a>) (WWDC 2011)</strong></p> <p><strong>Note: links require a free Apple Developer membership.</strong></p> <p>In <A HREF="http://stackoverflow.com/questions/12829705/programmatically-getting-the-date-next-sunday-at-5pm">this SO question</A>, 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:</p> <pre><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]; } } </code></pre> <p>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:</p> <pre><code>number of notifications = 1 Notification #1 =================== Body: LIVE SHOW REMINDER: The live show is about to start! Details: &lt;UIConcreteLocalNotification: 0x1d5f2200&gt; {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)} </code></pre> <p>What am I doing wrong here?</p>
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    1. COhey, any chance you were able to figure out the solution for this?
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