NSDateComponents returning strange hour

I'm writing some code to acquire images that live on a server. The image filenames are in yyyyMMdd_HHmm format (in UTC) and generated every 3 hours (1200,1500,1800,开发者_开发知识库..etc). When the code starts, I get the current UTC date and extract the time. I then create an NSDateComponent from this, and looking at the current time, set the hour of the NSDateComponent to the hour of the next image. Very strangely, if I set the hour to 3, the NSDate I get back from NSCalendar dateFromComponents has a different hour... what gives? (I have added in the console output as well).

// get current time in UTC
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
NSTimeZone *UTC = [NSTimeZone timeZoneWithName:@"UTC"];
[dateFormatter setTimeZone:UTC];

//  get the current hour & min
[dateFormatter setDateFormat:@"HHmm"];
NSDate *today = [NSDate date];
NSString *str = [dateFormatter stringFromDate:today];
NSInteger hour = [str integerValue];
NSLog(str);

// find next image hour
NSCalendar *cal = [[NSCalendar alloc]   initWithCalendarIdentifier:NSGregorianCalendar];
unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit;
NSDateComponents *currentImageDateComps = [cal components:unitFlags fromDate:today];

if (hour > 2100) {
    [currentImageDateComps setHour:0];
    // add an extra day as next image is start of new day
    [currentImageDateComps setDay:[currentImageDateComps day]+1];

} else if (hour < 300) {
    [currentImageDateComps setHour:3];
} else if (hour < 600) {
    [currentImageDateComps setHour:6];
} else if (hour < 900) {
    [currentImageDateComps setHour:9];
} else if (hour < 1200) {
    [currentImageDateComps setHour:12];
} else if (hour < 1500) {
    [currentImageDateComps setHour:15];
} else if (hour < 1800) {
    [currentImageDateComps setHour:18];
} else if (hour < 2100) {
    [currentImageDateComps setHour:21];
}
[currentImageDateComps setMinute:0];


NSLog(@"hour %d", [currentImageDateComps hour]);
NSLog(@"minute %d", [currentImageDateComps minute]);
// construct the date
NSDate *currentImageDate = [cal dateFromComponents:currentImageDateComps];
[dateFormatter setDateFormat:@"yyyyMMdd_hhmm"];
NSString *nextImage = [dateFormatter stringFromDate:currentImageDate];

self.currentFilename = [nextImage stringByAppendingString:@".png"];
NSLog(self.currentFilename);

The console output produced from the NSLogs is this. I get a value of 3 for hour but it gets returned as '1700'?

2010-01-25 12:25:02.049 Forecast[8447:207] viewDidLoad

2010-01-25 12:25:02.052 Forecast[8447:207] 0225

2010-01-25 12:25:02.052 Forecast[8447:207] hour 3

2010-01-25 12:25:02.053 Forecast[8447:207] minute 0

2010-01-25 12:25:02.053 Forecast[8447:207] 20100124_1700.png

(I'm aware there are some memory release issues but I want to get it functional first..)

A time zone issue maybe?

What's wrong with:

NSString *nextImage = [NSString stringWithFormat:@"%04d%02d%02d_%02d%02d.png", [currentImageDateComps year], [currentImageDateComps month], [currentImageDateComps day], [currentImageDateComps hour], [currentImageDateComps minute]];

and

self.currentFilename = nextImage;