In UWP Windows 10 apps, we can get the purchase date of the app or an add-on. (Documenation here)
var receipt = await CurrentApp.GetAppReceiptAsync();
But no were it's mentioned the date is in UTC or local user time! Thanks.
If you take a look at the receipt you will see that the date has 'Z' at the end:
PurchaseDate="2015-12-14T19:37:05.692Z"