the best by date means that yeast cells may start to die by that date....I've used white labs vials that were dated up to 11 months past, granted, you need to make a starter with the older vials and not pitch strait to the Wort. I've had no issues, no off flavors.
I wouldnt be concerned about the yeast....but its only 12 hrs old...give it a chance to pick up a bit.
BTW I like your diy stir plate, im considering making one myself in the near future you have plans for yours?
This is from white labs FAQ page.
What is the shelf life of White Labs Yeast?
Quick Answer: 4 Months
Long Answer: Yeast is a living organism. As such, it needs to be in the right conditions to survive. Dry yeast can stay alive for one year, but yeast left in liquid form, even though it's a better product in terms of taste and performance, is more perishable. At White Labs, we are constantly working on our recipe to maximize the viability of the yeast in long term storage. The longer we can make the yeast last in the vial, the better shape it will be in for fermentation. After 30 days in the vial, the viability of our yeast is 75-85%, which is very high for liquid yeast. Yeast that is harvested after a brewery fermentation will typically have a viability of less then 50% after 30 days. Our high viability is due to the health of the yeast and nutrient content of our liquid at packaging. After 6 weeks, lag time before active fermentation is usually between 15-20 hours. The shelf life for White Labs Yeast is four months. Yeast used after this point is usually fine, but lag times will be longer. There will be living yeast in most vials for 6-12 months, so if a starter is made to activate the yeast, successful fermentations can be carried out with aged yeast.