You need to check the EXIF properties dictionary:
CFDictionaryRef props = CGImageSourceCopyPropertiesAtIndex(source, 0, NULL);
NSDictionary *exif = [(NSDictionary *)props objectForKey : (NSString *)kCGImagePropertyExifDictionary];
NSString *dateTimeString = [exif objectForKey:(NSString *)kCGImagePropertyExifDateTimeOriginal];