How to check the first date an app is installed?

I have a paid app and want to offer it as free for a few days in the Store. But only for those who installed it during these special days I want to show ads.

How can I check if a user installed the app in a specific time range?

I thought of these solutions:

I could have a sign in previous packages and check it in this package. if it was not available this was the first install.
I could save the date after installation since the first time.

But these won't work now.

Thanks.

Solution

You can get App receipt from the store (here is how it can look like) and parse the PurchaseDate of your product. However this will return the date when the app has been first installed, if the user has reinstalled the app, then it will return the date of the very first installation (first 'purchase' from the store).

You can also have a value in LocalSettings that will be responsible for remembering the date of first run of the app. But this doesn't mean the date of installation - user can install the app and run it after a month. Also when user reinstalls the app, you will get the new value.