How do I calculate the number of days in this year in Objective C

Question

How can i calculate the number of days in a year for any calendar, not just gregorian. I have tried this

NSUInteger *days = [[NSCalendar currentCalendar] rangeOfUnit:NSDayCalendarUnit inUnit:NSYearCalendarUnit forDate:[NSDate date]];

but that gives me the number of days in the current month instead of the number of days in the current year.

Answer 1

I don't know objective-c but it is a simple algorithm to determine if it is a leap year.

if ( year % 400 == 0 )
   then 366 // Leap Year
else if ( year % 100 == 0 )
   then 365 // Non-Leap Year
else if ( year % 4 == 0 )
   then 366 // Leap Year
else
   365 // Non-Leap Year

Or if you want a little less verbose version

if ((year % 4 is 0) && (year % 100 != 0) || (year % 400 == 0))
   then 366 // Leap Year
else
   365 // Non-Leap Year

Answer 2

Maybe you can use the components:fromDate:toDate:options: selector which is meant to (and I quote official Apple docs): Returns, as an NSDateComponents object using specified components, the difference between two supplied dates. ?

Also read this post that clarifies the behaviour you are seeing.

Answer 3

I finally came up with a solution that works. What I do is first calculate the number of months in the year and then for each month calculate the number of days for that month.

The code looks like this:

NSUInteger days = 0;
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *today = [NSDate date];
NSDateComponents *components = [calendar components:NSYearCalendarUnit fromDate:today];
NSUInteger months = [calendar rangeOfUnit:NSMonthCalendarUnit
                                   inUnit:NSYearCalendarUnit
                                  forDate:today].length;
for (int i = 1; i <= months; i++) {
    components.month = i;
    NSDate *month = [calendar dateFromComponents:components];
    days += [calendar rangeOfUnit:NSDayCalendarUnit
                           inUnit:NSMonthCalendarUnit
                          forDate:month].length;
}

return days;

It is not as neat as I would have hoped for but it will work for any calendar such as the ordinary gregorian one or the islamic one.

Answer 4

In order to do what you want you have to find the range of days in a month and months in a year for a specific calendar. This function will do what you need for any calendar that uses day/month/year units (or has some equivalent mapped):

NSInteger getDaysInYear(NSDate* date)
{
    // Get the current calendar
    NSCalendar* c = [NSCalendar currentCalendar];

    // Find the range for days and months in this calendar
    NSRange dayRange = [c rangeOfUnit:NSDayCalendarUnit inUnit:NSYearCalendarUnit forDate:date];
    NSRange monthRange = [c rangeOfUnit:NSMonthCalendarUnit inUnit:NSYearCalendarUnit forDate:date];

    // Get the year from the suppled date
    NSDateComponents* yearComps = [c components:NSYearCalendarUnit fromDate:date];
    NSInteger thisYear = [yearComps year];

    // Create the first day of the year in the current calendar
    NSUInteger firstDay = dayRange.location;
    NSUInteger firstMonth = monthRange.location;
    NSDateComponents* firstDayComps = [[[NSDateComponents alloc] init] autorelease];
    [firstDayComps setDay:firstDay];
    [firstDayComps setMonth:firstMonth];
    [firstDayComps setYear:thisYear];
    NSDate* firstDayDate = [c dateFromComponents:firstDayComps];

    // Create the last day of the year in the current calendar
    NSUInteger lastDay = dayRange.length;
    NSUInteger lastMonth = monthRange.length;
    NSDateComponents* lastDayComps = [[[NSDateComponents alloc] init] autorelease];
    [lastDayComps setDay:lastDay];
    [lastDayComps setMonth:lastMonth];
    [lastDayComps setYear:thisYear];
    NSDate* lastDayDate = [c dateFromComponents:lastDayComps];

    // Find the difference in days between the first and last days of the year
    NSDateComponents* diffComps = [c components:NSDayCalendarUnit 
                                       fromDate:firstDayDate
                                         toDate:lastDayDate
                                        options:0];

    // We have to add one since this was subtraction but we really want to
    // give the total days in the year
    return [diffComps day] + 1;
}

If you want to specify this year, you can call it simply as getDaysInYear([NSDate date]);, or you could create a date from specific year/other components and pass that. You could also very easily re-implement it as a method call.