Date calculations without daylight savings time

Question

I'm trying to perform date calculations for a scheduling app but am running into issues with daylight savings time. I'm trying to determine the number of days between two dates with:

NSDateComponents *activityComponents = [[NSDateComponents alloc]init];
activityComponents = [self.calendar components:NSDayCalendarUnit
                                      fromDate:self.viewBeginDate
                                        toDate:item.ES
                                       options:0];
NSLog(@"Days: %d and hours: %d", [activityComponents day], [activityComponents hour]);

where self.viewBeginDate and item.ES are both NSDate objects and self.calendar is a gregorian calendar.

When calculating between Nov 1, 2013 and Nov 1, 2013, it provides 0.
When calculating between Nov 1, 2013 and Nov 2, 2013, it provides 1.
When calculating between Nov 1, 2013 and Nov 3, 2013, it provides 2.
When calculating between Nov 1, 2013 and Nov 4, 2013, it provides 2 again!

Here is my output:

    Days: 0 and hours: 0
    Days: 1 and hours: 0
    Days: 2 and hours: 0
    Days: 2 and hours: 24
    Days: 3 and hours: 23

I'd like to be able to determine the number of days between 2 NSDate objects without impact by Daylight Savings Time, as my app doesn't care what local or timezone the user is in. I just want it to tell me that there are 3 days between Nov 1, 2013 and Nov 4, 2013. Any thoughts?

My NSDate objects are created from strings in the format of: 2013-11-01 00:00:00 +0000. Each date has the time set to 00:00:00 with the GMT offset of +0000.

I've thought of two possible options:

Use isDaylightSavingTime to check each date and create a new (modified) date offset by 1 hour if it the date is Daylight Savings Time. I'm worried about the added calculation time as my app calculates the date between a given begin date and an array of other dates. This array may have up to 2,000 dates in it. I'm not sure if that's a legit concern or not...
create a custom timezone that is not affected by DST. Is this even possible?

I've changed my calendar to UTC with no difference in the results. Did I do this right? [self.calendar setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]]; — dfro
I've listed out the timezones with NSLog(@"%@", [NSTimeZone knownTimeZoneNames]); and got a long list in the likes of "America/such-and-such" and "Europe/such-and-such" and even GMT, but I didn't see UTC. Is it not always available? — dfro
Use GMT, it is the old name for UTC. I am amazed that Apple does not add UTC. — zaph

CRD CRD · Accepted Answer · 2013-11-25T18:09:14

I can confirm your observations using dates around the 27 October (when British Summer Time started) and setting the time zone to Europe/London. The "n days 24 hours" seems to be a quirk (kind word for a bug, somebody forgot to carry).

Solution 1: Use an NSCalendar set to UTC. UTC has no daylight savings.

Solution 2: As all your times are at midnight and the maximum daylight savings shift is 1 hours then a "day" is between 23 and 25 hours. To allow for this, and handle the quirk at the same time simply:

daysBetween = [activityComponents day] + ([activityComponents hour] >= 23 ? 1 : 0);

Note that this only works because time is the same in the two dates!

Date calculations without daylight savings time

1 Answers