I think what you need is to create an NSDateFormatter and call setDateStyle: to set it to NSDateFormatterNoStyle, and then it will only show the time without showing date.
NSString *originalString = @"01:23:45";
NSDateFormatter *fromFormatter = [[NSDateFormatter alloc] init];
[fromFormatter setDateFormat:@"HH:mm:ss"];
[fromFormatter setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] autorelease]];
[fromFormatter setTimeZone:[NSTimeZone systemTimeZone]];
NSDate *date = [fromFormatter dateFromString:originalString];
[fromFormatter release];
NSDateFormatter *toFormatter = [[NSDateFormatter alloc] init];
[toFormatter setDateStyle:NSDateFormatterNoStyle];
[toFormatter setTimeStyle:kCFDateFormatterFullStyle];
[toFormatter setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Tokyo"]];
[toFormatter setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:@"ja"] autorelease]];
NSString *s = [toFormatter stringFromDate:date];
NSLog(@"s:%@", s);
[toFormatter release];