It's the Red Sea Pro test kit? The expiry on the Salifert one is 2020? They essentially agree with each other, I'd believe them.

There is not a good DIY calcium standard you can make because calcium salts are hygroscopic, so you can't accurately tell how much water is in the CaCl when you weigh it. Maybe it would be "close enough" to get an idea (especially if you used a freshly opened tub of CaCl), but I wouldn't call it a "standard solution". The easiest and cheapest thing you can do is test your freshly mixed saltwater mixed at 1.0264 since each brand is usually predictable (unless you also just happened to have an off batch), and would provide you an answer that is at least as accurate as a homemade CaCl solution. Which salt do you use?

Just thought I would let you know that I have been using Salifert for a couple of years but couldn't get one for testing alk so I ended up getting Red Sea. I have a mixed reef tank that is doing well and I thought that I had my alk at about 8.5 but when I tested with red sea it was 7.6. I was quite surprised and was informed by another reefer that Salifert always tests a bit higher. I went and bought another new Salifert alk test kit as well and it was still off by about 0.6-0.8 so just a little food for thought!