Submit. · just now. Report Abuse. The best before date is based on a number of factors. The two most important are the rate at which the plastic container the water in leeches chemicals into the water, and the mineral content of the water and what effects light and heat may have on the water.Usually the best-before date is still at least one year after the water has been bottled.ubmit. · just now. Report Abuse. The best before date is based on a number of factors. The two most important are the rate at which the plastic container the water in leeches chemicals into the water, and the mineral content of the water and what effects light and heat may have on the water.