Why is there no use by date on lexmark ink

[iPud]

Hello to everyone. I am in need of finding out the reason behind no date or code on Lexmark ink cartridges. You see I work for a large department store and see other ink with warranty dates and some with codes. But if a cartridge is on the counter I have no way of knowing if it is dryed out. I have asked are main office about this and there answer was a joke or could be a shock. Ther answer "Lexmark ink cartridges need no date they are good until you open them".
So for the past three days I have been looking for some one to explain why only Lexmark ink cartridges have no use by date.
One reason I have found and I am starting to think it is true is that once the ink cartridge is sold from Lexmark to the store to resale they no longer liable for it to work it is the stores problem now. That to me dose not say much for the Lexmark company.
I know that in most cases the ink will be good for up to 2 years but if there is no date and the customer keeps taking the one in the back to think they are getting good ink after a while no one know if any are good.
I have been thinking of trying to go by copyright dates but here again I may be wrong. We have some with 2007 up to 2011 so that may be the only will to tell.
I know I will not tell the customer what are main office said.

 

[LexmarkListens]

The foil has a date code as does the cart, this shows when the cartridge was made. The shelf life is 2 years.

 

[imacardie]

The foil has a date code as does the cart, this shows when the cartridge was made. The shelf life is 2 years.

I have

The foil has a date code as does the cart, this shows when the cartridge was made. The shelf life is 2 years.

I have examined every part of the foil with a magnifying glass and there is no date ........unless it is coded. my box has no date but has manufacturers splash 'special OFFER 2 CARTRIDGES. SHOP CHARGED MY WIFE £50.00 !