Stockage dans les paramètres locaux de l'app.cs

public static DateTimeOffset? LastSeenPackageDate
{
    get
    {
        if (ApplicationData.Current.LocalSettings.Values
            .TryGetValue(nameof(LastSeenPackageDate), out object lastSeenDateStored)
            && lastSeenDateStored != null)
        {
            return DateTimeOffset.Parse(lastSeenDateStored.ToString());
        };
        return null;
    }
    set
    {
        ApplicationData.Current.LocalSettings
            .Values[nameof(LastSeenPackageDate)] = value?.ToString();
    }
}