The expiration date is pretty vague really. I personally don't use phytoplankton that's been sitting around for more than a few weeks.
The minute it's harvested it starts to die. Putting it in cold conditions slows this process.
In the case of DT's, it was probably chilled at the DT's warehouse, put in a box and shipped the cheapest way possible to a wholesaler (not refrigerated). Then, it probably sits for a while there (probably not refrigerated), before it's then shipped to the retailer slowly and sits a the doorstep of your local LFS. Then in turn I'm sure make it their upmost priority to load it into their refrigerator.
At which point, it sits, not being shaken, for a few months.
Then you take it home in your hot car and throw it in your cold fridge.
At what point are you expecting the majority of these die? During the shipping process I wager...
That's why I grow my own.