Hello,
Assuming that we are talking about common or garden Campus LAN switches attracting a Lifetime Warranty and not Data Centre class switches - I have found that you don't necessarily need the production dates and Serial numbers, all that you really need are two things -
1) The End of Sale for the particular model of switch
2) The End of Full Support date (occurs 5 years after End of Sale)
To the best of my knowledge:
After the "End of Sale" date the switch is still gets feature enhancements for five years - i.e. full support like it was still current...
Then after the 5 years is up after EoSale, the switch still continues to get both security fixes and the odd critical bug fix.
The hardware itself has a warranty for life as long as it is registered to you as the first owner.
I created a little chart last year as a reminder of when the Procurve / Provision models were moving between End of Sale and End of Full Support. Probably needs bringing up to date but it does aggregate a lot of the EoS stuff into a single page and give you an idea what i'm on about.
If someone wanted to do the same for the Comware switches or discovered an official summary page that could be useful. :-)
Don't forget the "Solved" and "Kudos - Thumbs up" buttons when posts are interesting, useful, helpful etc as they let others know about posts that are worth reading.
Thanks
Ian