I purchased Aquamira drops back in March of 2015. The product was stored indoors in climate controlled conditions. I opened the kit and used it on my AT section hike last October. One of the bottles has an expiration date of October 2018 on it. Does the expiration date apply to an unopened product? Or, does the expiration date still apply if the product has been used? I guess I am wondering if the shelf life is shortened once the drops have been opened and used. I want to take these drops with me as a "back up" for my upcoming section hike in early October.