I'm writing a GTD app for the iPhone. For the due tasks, I want to display something like "Due tomorrow" or "Due yesterday" or "Due July 18th". Obviously, I need to display "Tomorrow" even if the task is less than 24 hours away (e.g. the user checks at 11pm on Saturday and sees there's a task on Sunday at 8am). So, I wrote a method to get the number of days in between two dates. Here's the code...
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd-HH-mm"];
NSDate *nowDate = [dateFormatter dateFromString:@"2010-01-01-15-00"];
NSDate *dueDate = [dateFormatter dateFromString:@"2010-01-02-14-00"];
NSLog(@"NSDate *nowDate = %@", nowDate);
NSLog(@"NSDate *dueDate = %@", dueDate);
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *differenceComponents = [calendar components:(NSDayCalendarUnit)
fromDate:nowDate
toDate:dueDate
options:0];
NSLog(@"Days between dates: %d", [differenceComponents day]);
... and here's the output:
NSDate *nowDate = 2010-01-01 15:00:00 -0700
NSDate *dueDate = 2010-01-02 14:00:00 -0700
Days between dates: 0
As you can see, the method returns incorrect results. It should have returned 1 as the number of days between the two days. What am I doing wrong here?
EDIT: I wrote another method. I haven't done extensive unit tests, but so far it seems to work:
+ (NSInteger)daysFromDate:(NSDate *)fromDate inTimeZone:(NSTimeZone *)fromTimeZone untilDate:(NSDate *)toDate inTimeZone:(NSTimeZone *)toTimeZone {
NSCalendar *calendar = [NSCalendar currentCalendar];
unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
[calendar setTimeZone:fromTimeZone];
NSDateComponents *fromDateComponents = [calendar components:unitFlags fromDate:fromDate];
[calendar setTimeZone:toTimeZone];
NSDateComponents *toDateComponents = [calendar components:unitFlags fromDate:toDate];
[calendar setTimeZone:[NSTimeZone defaultTimeZone]];
NSDate *adjustedFromDate = [calendar dateFromComponents:fromDateComponents];
NSDate *adjustedToDate = [calendar dateFromComponents:toDateComponents];
NSTimeInterval timeIntervalBetweenDates = [adjustedToDate timeIntervalSinceDate:adjustedFromDate];
NSInteger daysBetweenDates = (NSInteger)(timeIntervalBetweenDates / (60.0 * 60.0 * 24.0));
NSDateComponents *midnightBeforeFromDateComponents = [[NSDateComponents alloc] init];
[midnightBeforeFromDateComponents setYear:[fromDateComponents year]];
[midnightBeforeFromDateComponents setMonth:[fromDateComponents month]];
[midnightBeforeFromDateComponents setDay:[fromDateComponents day]];
NSDate *midnightBeforeFromDate = [calendar dateFromComponents:midnightBeforeFromDateComponents];
[midnightBeforeFromDateComponents release];
NSDate *midnightAfterFromDate = [[NSDate alloc] initWithTimeInterval:(60.0 * 60.0 * 24.0)
sinceDate:midnightBeforeFromDate];
NSTimeInterval timeIntervalBetweenToDateAndMidnightBeforeFromDate = [adjustedToDate timeIntervalSinceDate:midnightBeforeFromDate];
NSTimeInterval timeIntervalBetweenToDateAndMidnightAfterFromDate = [adjustedToDate timeIntervalSinceDate:midnightAfterFromDate];
if (timeIntervalBetweenToDateAndMidnightBeforeFromDate < 0.0) {
// toDate is before the midnight before fromDate
timeIntervalBetweenToDateAndMidnightBeforeFromDate -= daysBetweenDates * 60.0 * 60.0 * 24.0;
if (timeIntervalBetweenToDateAndMidnightBeforeFromDate < 0.0)
daysBetweenDates -= 1;
}
else if (timeIntervalBetweenToDateAndMidnightAfterFromDate >= 0.0) {
// toDate is after the midnight after fromDate
timeIntervalBetweenToDateAndMidnightAfterFromDate -= daysBetweenDates * 60.0 * 60.0 * 24.0;
if (timeIntervalBetweenToDateAndMidnightAfterFromDate >= 0.0)
daysBetweenDates += 1;
}
[midnightAfterFromDate release];
return daysBetweenDates;
}
From the docs for
components:fromDate:toDate:options:
:Since the difference is less than a full day, it correctly returns a result of 0 days.
I am using this piece of code, it is working very well:
If all you care about is tomorrow or yesterday vs. a specific date, then you can save yourself a lot of work and just test whether the dates are only one calendar day apart.
To do that, compare the dates to find which is earlier and which is later (and if they compare equal, bail out with that result), then test whether 1 day after the earlier date produces a date with the same year, month, and day-of-month as the later date.
If you really do want to know exactly how many calendar days there are from one date to the other:
components:fromDate:
message to get the year, month, and day-of-the-month of the first date.abs
(see abs(3)) to take the absolute value.If they are not in the same year and month, test whether they are in adjacent months (e.g., December 2010 to January 2011, or June 2010 to July 2010). If they are, add the number of days in the earlier date's month (which you can obtain by sending the calendar a
rangeOfUnit:inUnit:forDate:
message, passingNSDayCalendarUnit
andNSMonthCalendarUnit
, respectively) to the day-of-month of the later date, then compare that result to the earlier date's day-of-month.For example, when comparing 2010-12-31 to 2011-01-01, you would first determine that these are in adjacent months, then add 31 (number of days in 2010-12) to 1 (day-of-month of 2011-01-01), then subtract 31 (day-of-month of 2010-12-31) from that sum. Since the difference is 1, the earlier date is one day before the later date.
When comparing 2010-12-30 to 2011-01-02, you would determine that they are in adjacent months, then add 31 (days in 2010-12) to 2 (day-of-month of 2011-01-02), then subtract 30 (day-of-month of 2010-12-30) from that sum. 33 minus 30 is 3, so these dates are three calendar days apart.
Either way, I strongly suggest writing unit tests at least for this code. I've found that date-handling code is among the most likely to have subtle bugs that only manifest, say, twice a year.
One thing you might try is using
rangeOfUnit:
to zero out hours, minutes and seconds from the start and end dates.In this example start will be
2010-06-19 00:00:00 -0400
, end will be2010-06-20 00:00:00 -0400
. I'd imagine this would work better with NSCalendar's comparison methods, although I haven't tested it myself.{
}
You can easily work it out using the very nice answer to this related question