NSString
NSString *time = [[timeString stringByReplacingOccurrencesOfString:@"T" withString:@" "] substringToIndex:19];
NSMutableString
NSMutableString *string = [[NSMutableString alloc]initWithString:timeString] ;
NSRange range = NSMakeRange(0, [string length]);
[string replaceOccurrencesOfString:@"T" withString:@" " options:NSCaseInsensitiveSearch range:range];