I started using using liquid chlorine last summer, with great success. Because I have mobility problems, I am more interested in convenience in dosing than in price. So, I use Kem Tek 10% liquid chlorine, which I buy at whichever big box store has the freshest stuff; it costs about $3.50/gal.
Last summer I was told at a pricey local pool maintenance shop that liquid chlorine starts to deteriorate after about 30 days. A tech at Kem Tek told me how to find the production run date on the carton: it is displayed on one side of the box as a Julian date. So, when I shop, I try to find the freshest stuff, most times it is within 45 days of production, and I only buy a week supply. In my testing, I believe I find that my chlorine level is affected by the age of the product.
In shopping I have found the following:
1. The stores keep the old stuff out front, like milk at the grocery
2. The cartons are almost always set up so the side with the date faces toward the back
3. Personnel profess to have no idea about shelf life, Julian dates, etc.
Does this make sense to other users?