What The Wrench and Soufreak have said above is true but what I think you're asking is this:
You have two skins for an aircraft, Skin1 and Skin2.
You want Skin1 until a certain date with Skin2 unavailable for selection.
Then after that date, you only want to be able to select Skin2.
If this is true, the above method won't work and the only way you will be able to do it is by creating two different aircraft folders with only one skin in each. Then you will have to change the service dates of the aircraft in the _DATA.ini as required for when you want the skins available.
All of this aside, the StartDefaultDate as detailed above, will work for you. The AI aircraft and the default skin of your flight will be the right skin according to the dates you set except that you will be able to change the skin manually.
I think this is a much better solution and an acceptable trade off to having an extra aircraft folder just to separate the skins.
Dels