Likely a stupid noob question regarding yeast

Others will have greater experience than I to give more details. However, with yeast age you are going to lose viable yeast cells. Many online yeast pitch calculators take into account the age of the yeast; so the longer you wait, the more die. Does losing some yeast cells matter, well in depends. Bigger beers need more cells. Simply put, the idea is to keep yeast happy. Don't shock them with big shifts in environments, either by pitching them into big beers without lots of help from friends or changing temp or gravity on them. Of course more details are needed to give you a definitive answer, but if it's a low gravity where you don't need a big pitch rate, may not be any issue what so ever. You do have a couple options if you want to take what you have and increase pitch rate. First as you've already stated you can harvest. Or as a second option, you can build a starter to up the count.

 

For one gallon batches, you're certainly NOT going to need the whole thing, or you'd be way over pitching. Obviously you either waste it by chunking out the excess, or save it for the next batch. I'd save it. I would also make sure you sanitize whatever you cut it open with, and sanitize a piece of foil, wrap it in that, then tape it closed.

Besides, how bad can a worst case scenario be for a one gallon batch?

 

For a 1G batch, pitch one level teaspoon (there are ~4 tsp / 11gm package)
Close the package tightly then seal with a short piece of painter's tape or a rubber band.
Store in fridge.
Shelf life is the expiration date on the package.

 

I'd tape it completely shut and seal the yeast package in a zip-lock bag. I'd also hurry along and make sure I use the rest of that yeast ASAP. Better safe than sorry here.

My general rule of thumb here is that if I can't use this yeast within a month, I'll either use the whole sachet of yeast or I'll throw the rest out (after that month).

 

The shelf life of yeast is much, much longer than liquid. IIRC, the date of Safale yeast is two years after manufacture.