Converts an ISO8601 duration (http://en.wikipedia.org/wiki/ISO_8601#Durations) to seconds (NSTimeInterval)

gistfile1.m

- (NSTimeInterval) timeIntervalWithISO8601duration:(NSString *)duration
{
    // duration is in the format "P3Y6M4DT12H30M5S" OR "P2W"
    // The P simply indicates that this is a duration, this should be the first character.
    // The T is the separator between the date portion (day, month, year) and the time portion (hour, minute, second).
    
    if ([duration characterAtIndex:0]!='P') return 0;
    
    NSScanner *iso8601duration = [NSScanner scannerWithString:duration];
    iso8601duration.charactersToBeSkipped = [NSCharacterSet characterSetWithCharactersInString:@"P"];
    
    NSTimeInterval seconds = 0;
    BOOL isTimePortion = NO;
    while (!iso8601duration.isAtEnd)
    {
        double value;
        NSString *units;
        char cUnit;
        
        if ([iso8601duration scanDouble:&value])
        {
            if ([iso8601duration scanCharactersFromSet:[NSCharacterSet uppercaseLetterCharacterSet] intoString:&units])
            {
                for (int i=0; i<units.length; ++i)
                {
                    cUnit = [units characterAtIndex:i];
                    
                    switch (cUnit)
                    {
                        case 'Y': seconds+=31557600*value;  break;
                        case 'M': seconds+=
                            isTimePortion ? 60*value 
                            : 2629800*value;  break;
                        case 'W': seconds+=604800*value;    break;
                        case 'D': seconds+=86400*value;     break;
                        case 'H': seconds+=3600*value;      break;
                        case 'S': seconds+=value;           break;
                        case 'T': isTimePortion=YES;        break;
                    }
                }
            }
        }
        else break;
    }
    
    return seconds;
}

Thanks! Used this to parse duration strings from the YouTube API for a Cocoa app. One minor bug - durations that begin with "PT", like YouTube's, always return 0 because the first conditional fails. Fixed like so:

--- snip --- 
    NSString *units;
    char cUnit;

        if ([iso8601duration scanCharactersFromSet:[NSCharacterSet characterSetWithCharactersInString:@"T"] intoString:&units]){
            isTimePortion = YES;
        }

        if ([iso8601duration scanDouble:&value]){
--- snip ---