Describe the process of creating a custom variable template to handle complex data transformations.

Creating a custom variable template in Google Tag Manager (GTM) to handle complex data transformations involves several steps within the GTM Template Editor. The template editor allows creating reusable custom variables and custom tags. First, access the Template Editor by navigating to 'Templates' in the GTM interface and selecting 'Variable Templates' > 'New'. Define template information. In the 'Info' tab, provide a name for the template, a description of its purpose, and an author name. This information will help other users understand the template and its functionality. Define template fields. In the 'Fields' tab, define the input fields that the template will require. These fields allow users to configure the variable with specific values. Field types include text, number, boolean, select, and URL. For complex data transformations, you might need multiple input fields to capture all the necessary data. For example, if the template transforms a date string from one format to another, you might need input fields for the input date format and the output date format. Write the template code. In the 'Code' tab, write the JavaScript code that performs the data transformation. This code will access the input fields defined in the 'Fields' tab and use them to transform the data. The template code must be written in a sandboxed JavaScript environment that restricts access to certain browser APIs. Use the GTM template APIs to access data, perform calculations, and return the transformed value. For example, you can use the `data` object to access the input field values and the `templateStorage` API to store data between template executions. For date formatting, you can use the `formatDate` API. Test the template. In the 'Test' tab, test the template with different input values to ensure that it performs the data transformation correctly. Use the debug console to log the input values, the transformed value, and any errors that occur during the transformation. Publish the template. Once you have tested the template and are confident that it is working correctly, publish it to the GTM container. After publishing the template, you can use it to create custom variables in GTM. Select the custom template from the list of variable types and configure the input fields with the appropriate values. Finally, consider template security. Adhere to all security guidelines to avoid malicious use. Validate all third-party code, and only use Template Gallery templates from trusted sources.