Azure Using shared access signatures for blob storage

I'm trying to implement shared access signatures when saving blobs (pdf files) to azure blob storage. I want the link to the pdf file to expire after a set time, but it doesn't seem to be working.

The pdf creation and save process works fine, I create a pdf file and upload it to azure blob storage. I can retrieve the blob URL and if I paste it into a browser, the pdf report shows up ok. It never expires though.

I set the expiry time to 2 minutes while Im testing (it would be around 24 hours in production). I can continue to view the report, nothing stops me.

I am new to shared signature access, but from what I've found so far, it is supposed to stop access after the specified time(is this correct?).

This is how I create the storage details (in the constructor of my class):

public BlobService()
{
    //use for local development testing
    _connectionString = Settings.AzureWebJobsStorage;
    this._container = Settings.ReportBlobContainer;

    try
    {
        storageAccount = CloudStorageAccount.Parse(_connectionString);
    }
    catch (StorageException e)
    {
        throw;
    }

    // Get an account SAS token.
    string sasToken = GetAccountSASToken();

    // Use the account SAS token to create authentication credentials.
    StorageCredentials accountSAS = new StorageCredentials(sasToken);

    var blobClient = storageAccount.CreateCloudBlobClient();

    chpBlobContainer = blobClient.GetContainerReference(this._container);
    // Get the URI for the container.
    Uri containerUri = GetContainerUri();
    chpBlobContainer = new CloudBlobContainer(containerUri, accountSAS);

    try
    {
        if (chpBlobContainer.CreateIfNotExists())
        {
            //leave the access to private only (default)
            // Enable public access on the newly created container.
            //chpBlobContainer.SetPermissions(
            //    new BlobContainerPermissions
            //    {
            //        PublicAccess = BlobContainerPublicAccessType.Blob
            //    });
        }
    }
    catch(Exception ex)
    {
        var tmp = ex.Message;
    }

}

and this is how I generate the SAS token

private string GetAccountSASToken()
{
    // Retrieve storage account information from connection string
    //CloudStorageAccount storageAccount = Common.CreateStorageAccountFromConnectionString();

    // Create a new access policy for the account with the following properties:
    // Permissions: Read, Write, List, Create, Delete
    // ResourceType: Container
    // Expires in 24 hours
    // Protocols: HTTPS or HTTP (note that the storage emulator does not support HTTPS)
    SharedAccessAccountPolicy policy = new SharedAccessAccountPolicy()
    {
        // When the start time for the SAS is omitted, the start time is assumed to be the time when the storage service receives the request. 
        // Omitting the start time for a SAS that is effective immediately helps to avoid clock skew.
        //Permissions = SharedAccessAccountPermissions.Read | SharedAccessAccountPermissions.Write | SharedAccessAccountPermissions.List | SharedAccessAccountPermissions.Create | SharedAccessAccountPermissions.Delete,
        Permissions = SharedAccessAccountPermissions.Read | SharedAccessAccountPermissions.Write | SharedAccessAccountPermissions.Create,
        Services = SharedAccessAccountServices.Blob,
        ResourceTypes = SharedAccessAccountResourceTypes.Container | SharedAccessAccountResourceTypes.Object,
        //SharedAccessExpiryTime = DateTime.UtcNow.AddHours(24),
        //just for testing the expiry works
        SharedAccessExpiryTime = DateTime.UtcNow.AddMinutes(2),
        Protocols = SharedAccessProtocol.HttpsOrHttp
    };

    // Create new storage credentials using the SAS token.
    string sasToken = storageAccount.GetSharedAccessSignature(policy);

    // Return the SASToken
    return sasToken;
}

I can see the blob from the Azure storage explorer, so the connection and generation process is fine, its just the expiry.

Can anyone help me out? I am clearly doing something wrong here.