Is it illeagal to sell out of date stock and what legislation governs it?

Answer 1

In the UK, the Relevant or Related Legislation: Sale of Goods Act 1979. Supply of Goods and Services Act 1982. Sale and Supply of Goods Act 1994. The Sale and Supply of Goods to Consumers Regulations 2002.

Key Facts:

• Wherever goods are bought they must "conform to contract". This means they must be as described, fit for purpose and of satisfactory quality (i.e. not inherently faulty at the time of sale).

• Goods are of satisfactory quality if they reach the standard that a reasonable person would regard as satisfactory, taking into account the price and any description.

• Aspects of quality include fitness for purpose, freedom from minor defects, appearance and finish, durability and safety.

• It is the seller, not the manufacturer, who is responsible if goods do not conform to contract.

• If goods do not conform to contract at the time of sale, purchasers can request their money back "within a reasonable time". (This is not defined and will depend on circumstances)

Most countries would have their equivalent of these legislation.

Answer 2

Not sure what country you're in, so I'll answer you UK.
In uk, there are 2 types of dates:
Use by Date
Best before date
A use by date is always specified as a specfific day, after which it must not be sold, and should not be consumed. (stuff like meat, daily products etc)
A best before date may be a particular month, rather than date, such as 'August 2007'.
It's not actually illegal to sell a product past it's best before date, and is often done, however, it IS illegal to remove, obscure or change the date. In additon, there is legislation which states that food must be fit for consumption, and it is often the case that food past it's best before date is no longer fit for consumption.
as I say, this is for the UK, but don't expect it to differ hugely between countries