USING ESSENTIAL OILS
Most essential oils do not last forever!
This recommendation is particularly true to the zest of citrus oils (Bergamot, Lemon, Mandarin, Orange, Grapefruit, Tangerine ...) that can be used for about 12 to 24 months, depending on storage conditions.
On the other hand, some essential oils such as frankincense, sandalwood, the myrrhre, patchouli will "mature" over time and get better.