Calculate dates based on specific time zone

Using NSDate is tricky to me. I know it represents a specific point in time, not tied to a specific time zone or locale. I've also read a lot fo the docs on NSDateFormatter, NSCalendar, NSDateComponents, NSlocale, and NSTimeZone.

I'm trying to do calculations based on two specific Pacific time zones. For example, I want to be able to count the number of days from date A to date B. I don't want to 'hack' it by parsing strings, as I want things like DST to be calculated as well.

Can anyone recommend a way to do this?

Edit: Found solution:

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSTimeZone *zone = [NSTimeZone timeZoneWithName:@"PST"];
[calendar setTimeZone:zone];
NSDateComponents *comp = [calendar components:(NSYearCalendarUnit开发者_高级运维 | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate:date];
date = [calendar dateFromComponents:comp];
[calendar release];

This results in a date localized to Pacific time. I know this causes problems with different locales, but my app is unique, and this is justified.

I can then compare dates using the division by 86400 as was posted.

Although, it seems like overkill - a lot of extra work - perhaps there is a shorter way to get a date with a specific time zone?

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You could also convert both NSDate objects to GMT. Like this:

NSDate *date;
NSTimeZone *timeZone;
NSDate *gmtDate = [date dateByAddingTimeInterval:-[timeZone secondsFromGMTForDate:date]];

And then set the NSCalendar's time zone to GMT. Like this:

NSCalendar *calendar;
[calendar setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0];

You can then use the calendar and your GMT dates to calculate whatever you need to know.

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I suppose you could calculate the difference between the time zones. Like this:

NSDate *date1;
NSDate *date2;
NSTimeInterval dateDifference = [date1 timeIntervalSinceDate:date2];

Then calculate the time zone difference. Like this:

NSTimeZone *timeZone1;
NSTimeZone *timeZone2;
// Assuming date1 is in timeZone1...
NSTimeInterval timeZoneDifference = [timeZone1 secondsFromGMTForDate:date1] - [timeZone2 secondsFromGMTForDate:date2];

Then sum the two. Like this:

NSTimeInterval difference = dateDifference + timeZoneDifference;

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You should create two NSDate instances that represent the two dates you want to compare. To do this, you can set up NSDateComponents instances and set not only their day/month/year/hour but also their time zone (-[NSDateComponents setTimeZone:]). Then call -[NSDateComponents date] to convert them to NSDate.

[date1 timeIntervalSinceDate:date2] then gives the difference (in seconds) between the two dates. Divide the result by 86,400 to get the number of days.

Alternatively, you can call [NSCalendar components:fromDate:toDate:options:] with the NSDateComponent instances directly to get an NSDateComponents with the difference between the two.