and another thing! some seed I have says it was packed in 2010(just bought) not to expire until 2012,does that mean it is old already?or does old seed mean it ages once opened? sorry! I hope someone understands what I'm on about!
Packing date = usually the year the seed was packed by the seed seller (not necessarily when the seed was collected and processed ...) So yes, your seed may be even older than the date stamped. If you think about it, some packing dates just do not coincide with usual harvesting dates.
Use by date = the recommended date
suggested by the seed seller to guarantee the seed will give optimal germination This does not mean 100% germination as there will always be some reason why some seed won't germinate - so if you buy a packet of 500 seeds, the "guarantee" usually means about just over 50-60% germination. Some seed will far exceed the use by date - tomatoes, brassicas and things like lettuce and carrots have a much longer viability than the recommended use by date. Parsnip seed does not.
Batch number - this number enables the seed seller to trace back where he got his seed from, and that merchant should also have records where the seed originated and when it was harvested. All part of the seed legislation which used to be confined to farming of grain but since 2011 includes all edible crop seeds.
"Old" seed can still be perfectly fine as there are several factors that determine viability. Age is an obvious one, but storage conditions play a vital role (extreme heat and dampness such as in the tin box in the shed or greenhouse are not good storage conditions) and also air. Oxygen will age seed quickly so exclusion of air (such as the vacuum sealed packs) will allow seed to store for much longer. I can honestly say that I sowed an entire packet of seed from what was an unopened pack that was 10 years past its use by date expecting nothing, and all the seed germinated ....
Not what I was expecting but is a good example of how well some seed can keep.