Depending on where you buy it a
battery purchased today could
already be over a year old.
Shelf life starts ticking the day it is manufactured and not related to when you purchased it.
Even when intelligently self-discharged DJI recommends charging them up every 3 months or so. So
batteries warehoused for long periods may be compromised to some degree.
Batteries purchased direct from DJI may be newer because not passed through the distribution chain (but DJI sales only 10-15% discount).
For that reason I'd recommend not purchasing
battery until you need it. They are in good supply and frequently on sale elsewhere. They rarely sell below$45 unless on clearance (and those are likely the oldest
batteries or fakes).
Saw a post (Spark 317) that Target is selling off inventory for $34. Might be older, but I'm sure they are legit DJI
batteries. Out of stock most places - but going to pick one up I think as a backup, since pretty cheap.