Basically, the best-before date on the product always applies. This is only valid when unopened and should not be exceeded. Use beyond this date may result in undesirable side effects or even a complete lack of effect of the vitamin D3 drops.
The manufacturer is responsible for the safety and proper use of the product until the best-before date has been reached. Food supplements that consist of fat-soluble substances (such as vitamin D3) can easily become rancid and should therefore not be used beyond the expiry date. If you are nevertheless over the MDH, you can delay the consumption of vitamin D3 drops by up to two months by ideal storage.