Following a discussion opin another thread about dry yeast dating, I contacted a friend at Lallemnd. Here's his reply. BTW, I asked him for a copy of the chart he mentions and will post it here when I get it.
Best By date is the point where you start to see a reduction in the performance of the yeast. The lab geeks have figured this out so they call it probably way before they should. Treated well…..that sachet will still perform for years. Strains are different too…..some have a shorter life than others….I have a chert somewhere…I can send to ya.