When you buy seeds in the UK they will have a 'use by' date on the packet. Often they will be in a sealed foil packet within the main packet. This excludes air and light which keeps the seed fresh.

Unfortunately many seeds are supplied in quantities way beyond that required for the average garden or allotment for one year.

Continued @ allotment.org.uk

Note : this page is from a North Carolina website so you need to take into account local seasons

Basil (Ocimum basilicum L. and its varieties) is a popular herb known for its flavorful foliage. The fresh or dried leaves add a distinctive flavor to many foods, such as Italian style tomato sauces, pesto sauce and salad dressing. The essential oils and oleo-resins may be extracted from leaves and flowers and used for flavoring in liqueurs and for fragrance in perfumes and soaps.

Continued @ ncsu.edu