Heat and daylight age a fragrance faster than time does. Simple rules for keeping a flacon exactly as it left the atelier.
A sealed flacon does not really expire. It oxidises — and oxidation is driven by three things you control: light, heat and air. Manage those and a bottle will still be true years after you opened it.
The bathroom is the worst room in the house for it. Shower heat cycles the temperature several times a day, and each cycle pushes the composition a little further along. A drawer or a closed cupboard in an air-conditioned room is the whole solution.
Direct sun is the fastest way to ruin a citrus opening. Ultraviolet light breaks down the lightest molecules first, so a bottle left on a windowsill loses its top notes long before anything else changes. Keep the outer carton; it was designed for exactly this.
Air matters more as the bottle empties. The headspace above the liquid fills with oxygen, so the last quarter of a bottle ages faster than the first three quarters did. If you have a flacon you wear rarely, decant part of it into a smaller atomiser.
One thing not to do: refrigerate. The temperature swings on the way in and out of the fridge do more harm than a stable 22°C cupboard ever will.