As an AI language model, I do not have access to your specific Excel add-in or its trial period validation mechanism. However, I can provide you with general guidelines on how to validate a trial period in C#:

Store the installation date of your software in a configuration file or database.
Calculate the number of days that have elapsed since the installation date using the current date.
Compare the number of days elapsed to the trial period duration.
If the number of days elapsed is greater than or equal to the trial period duration, disable the functionality of the software or prompt the user to purchase a license.
Here is an example C# code snippet that demonstrates this approach:

DateTime installationDate = GetInstallationDateFromConfig();
int trialPeriodDuration = 30; // days
int daysElapsed = (int)(DateTime.Now - installationDate).TotalDays;

if (daysElapsed >= trialPeriodDuration)
{
    // Disable functionality or prompt user to purchase license
}

Note that this is just a simple example and may need to be adapted to fit the specific needs of your software.
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