using System;
using System.Net;

class TrialValidation
{
    private static string serverUrl = "https://example.com/validate.php";

    public static bool IsTrialValid()
    {
        try
        {
            using (var client = new WebClient())
            {
                string response = client.DownloadString(serverUrl);
                DateTime trialEndDate = DateTime.Parse(response);

                if (DateTime.Now < trialEndDate)
                {
                    return true;
                }
            }
        }
        catch (Exception ex)
        {
            // handle any exceptions here
        }

        return false;
    }
}

In this example, we use a server-based validation system to validate the trial period of the Excel add-in. The IsTrialValid() method sends a request to a PHP script hosted on a remote server, which returns the end date of the trial period in the format "yyyy-MM-dd". The method then compares the current date to the trial end date and returns true if the trial is still valid.

You would need to replace the serverUrl variable with the URL of your PHP script that returns the trial end date.

Note that this is just a simple example, and you should implement additional measures to ensure that the validation system is secure and cannot be easily bypassed.
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