iPhone: method to calculate days till next birthday not accurate --? - iphone

Question

Here's a puzzler. I use the following to calculate the number of days between today's date and an upcoming birthday:
-(int) daysTillBirthday: (NSDate*)aDate {
// check to see if valid date was passed in
//NSLog(#"aDate passed in is %#",aDate);
if (aDate == nil) {
//NSLog(#"aDate is NULL");
return -1; // return a negative so won't be picked in table
}
//** HOW MANY DAYS TO BDAY
NSDate *birthDay = aDate; // [calendar dateFromComponents:myBirthDay];
//NSLog(#"birthDay: %#, today: %#",birthDay, [NSDate date]);
NSCalendar *calendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *thisYearComponents = [calendar components:NSYearCalendarUnit fromDate:[NSDate date]];
NSDateComponents *birthDayComponents = [calendar components:NSMonthCalendarUnit|NSDayCalendarUnit fromDate:birthDay];
[birthDayComponents setYear:[thisYearComponents year]];
NSDate *birthDayThisYear = [calendar dateFromComponents:birthDayComponents];
//NSLog(#"birthDayThisYear: %#",birthDayThisYear);
NSDateComponents *differenceHours = [calendar components:NSHourCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
NSDateComponents *differenceDays = [calendar components:NSDayCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
// NSLog(#"difference days: %i, hours %i",[differenceDays day],[differenceHours hour]);
//*** I added this to try and correct the "error" ***
if ([differenceDays day] == 0) { // is it today, or tomorrow?
if (([differenceHours hour] <= 0) && ([differenceHours hour] >= -24)) { // must be today
//NSLog(#"TODAY");
return (0);
[calendar release];
}else if (([differenceHours hour] >= 0) && ([differenceHours hour] <= 24)) {
//NSLog(#"TOMORROW");
return (1);
[calendar release];
}
}
if ([differenceDays day] < 0) {
// this years birthday is already over. calculate distance to next years birthday
[birthDayComponents setYear:[thisYearComponents year]+1];
birthDayThisYear = [calendar dateFromComponents:birthDayComponents];
differenceDays = [calendar components:NSDayCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
}
return ([differenceDays day]);
[calendar release];
}
Everything works, but the results are not accurate! I often find that birthdays that are close to today, but one day apart, result in [differenceDays day] being the same! i.e. if today is 6/6/2011 and I have two birthdays, one on 6/7/2011 and another 6/8/2011, then they are both shown as 1 day away!
Anyone have any better methods for accurately calculating this, or can spot the problem?
Many thanks.

Answer 1

NSCalendar provides a much easier way to do this:
NSDate *birthday = ...; // the birthday
NSDate *today = [NSDate date];
NSCalendar *c = [NSCalendar currentCalendar];
NSInteger birthdayDayOfYear = [c ordinalityOfUnit:NSDayCalendarUnit inUnit:NSYearCalendarUnit forDate:birthday];
NSInteger todayDayOfYear = [c ordinalityOfUnit:NSDayCalendarUnit inUnit:NSYearCalendarUnit forDate:today];
NSInteger different = birthdayDayOfYear - todayDayOfYear;
Basically, we're figuring out how far into the year today and the target date are (ie, today [5 Jun] is the 156th day of the year), and then subtract them to figure out how many days are in between them.
This method, of course, relies on the assumption that the target date is in the same year as the current date. I think it'd be fairly easy to work around that, however.
Another, even easier way to do this that will account for multi-year differences is like this:
NSDateComponents *d = [[NSCalendar currentCalendar] components:NSDayCalendarUnit fromDate:today toDate:birthday options:0];
NSInteger difference = [d day];
If you need to make sure that the birthday is in the future, that's easily accomplished as well:
NSDateComponents *year = [[[NSDateComponents alloc] init] autorelease];
NSInteger yearDiff = 1;
NSDate *newBirthday = birthday;
while([newBirthday earlierDate:today] == newBirthday) {
[year setYear:yearDiff++];
newBirthday = [[NSCalendar currentCalendar] dateByAddingComponents:year toDate:birthday options:0];
}
//continue on with the 2-line calculation above, using "newBirthday" instead.
update I updated the loop above to always increment from the original date n years at a time, instead of year-by-year. If someone is born on 29 Feb, incrementing by one year would yield 1 Mar, which would be wrong once you got to a leap year again. By jumping from the original date each time, we don't have this issue.

Answer 2

I do the exact same thing in one of my apps. Here is how I do it:
//This is the date your going to - in your case the birthday - note the format
NSString *myDateAsAStringValue = #"20110605";
// Convert string to date object
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:#"yyyyMMdd"];
NSDate *newDate = [dateFormat dateFromString:myDateAsAStringValue];
NSDateComponents *dateComp = [[NSDateComponents alloc] init];
NSCalendar *Calander = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *comps=[[NSDateComponents alloc] init];
unsigned int unitFlags = NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
dateComp = [Calander components:unitFlags fromDate:[NSDate date]];
[dateFormat setDateFormat:#"dd"];
[comps setDay:[[dateFormat stringFromDate:[NSDate date]] intValue]];
[dateFormat setDateFormat:#"MM"];
[comps setMonth:[[dateFormat stringFromDate:[NSDate date]] intValue]];
[dateFormat setDateFormat:#"yyyy"];
[comps setYear:[[dateFormat stringFromDate:[NSDate date]] intValue]];
[dateFormat setDateFormat:#"HH"];
[comps setHour:05];
[dateFormat setDateFormat:#"mm"];
[comps setMinute:30];
NSDate *currentDate=[Calander dateFromComponents:comps];
dateComp = [Calander components:unitFlags fromDate:newDate];
[dateFormat setDateFormat:#"dd"];
[comps setDay:[[dateFormat stringFromDate:newDate] intValue]];
[dateFormat setDateFormat:#"MM"];
[comps setMonth:[[dateFormat stringFromDate:newDate] intValue]];
[dateFormat setDateFormat:#"yyyy"];
[comps setYear:[[dateFormat stringFromDate:newDate] intValue]];
[dateFormat setDateFormat:#"HH"];
[comps setHour:05];
[dateFormat setDateFormat:#"mm"];
[comps setMinute:30];
NSDate *reminderDate=[Calander dateFromComponents:comps];
NSTimeInterval ti = [reminderDate timeIntervalSinceDate:currentDate];
int days = ti/86400;
return days;

Answer 3

I think I have found a solution. Checking the output carefully, it appears to all come down to the difference in HOURS. For example: comparing today with tomorrow's date might end up being, say, 18 hours away. This results in [difference day] being set at 0 i.e. it thinks tomorrow is today because it is less than 24 hours away.
You can see the fix below. I take the number of hours e.g. 18 and divide by 24 (to get the number of days). In this case 18/24 = 0.75. I then round this up i.e. to "1." So while [difference days] thinks tomorrow is today, by rounding up the hours, you know it is in fact tomorrow.
-(int) daysTillBirthday: (NSDate*)aDate {
// check to see if valid date was passed in
//NSLog(#"aDate passed in is %#",aDate);
if (aDate == nil) {
//NSLog(#"aDate is NULL");
return -1; // return a negative so won't be picked in table
}
//** HOW MANY DAYS TO BDAY
NSDate *birthDay = aDate; // [calendar dateFromComponents:myBirthDay];
NSCalendar *calendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *thisYearComponents = [calendar components:NSYearCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit fromDate:[NSDate date]];
NSDateComponents *birthDayComponents = [calendar components:NSMonthCalendarUnit|NSDayCalendarUnit fromDate:birthDay];
NSInteger timeNow = [thisYearComponents hour];
[birthDayComponents setYear:[thisYearComponents year]];
[birthDayComponents setHour:timeNow];
NSDate *birthDayThisYear = [calendar dateFromComponents:birthDayComponents];
//NSLog(#"today %#, birthday %#",[NSDate date],birthDayThisYear);
NSDateComponents *difference = [calendar components:NSDayCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
NSDateComponents *differenceHours = [calendar components:NSHourCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
double daysFromHours = ((double)[differenceHours hour])/24; // calculate number of days from hours (and round up)
int roundedDaysFromHours = ceil(daysFromHours);
NSLog(#"daysFromHours %.02f, roundedDaysFromHours %i",daysFromHours,roundedDaysFromHours);
if ([difference day] < 0) {
// this years birthday is already over. calculate distance to next years birthday
[birthDayComponents setYear:[thisYearComponents year]+1];
birthDayThisYear = [calendar dateFromComponents:birthDayComponents];
difference = [calendar components:NSDayCalendarUnit fromDate:[NSDate date] toDate:birthDayThisYear options:0];
}
//NSLog(#"%i days until birthday", [difference day]);
return (roundedDaysFromHours);
[calendar release];
}