Current year

Starting in v7.7 of Yoast, we’ve deprecated the %%currentyear%% variable (along with some others) as we see no valid use-case for them. As we don’t recommend using %%currentyear%%, a similar previous year variable is not available.

That being said, you or your developer can use the wpseo_register_extra_replacements filter to create your own custom variables.

An example code snippet can be found here:
https://gist.github.com/amboutwe/550c10ede7d065d9264930f5480ca748#file-yoast_seo_title_custom-variable-php

Typically code snippets are added to your theme’s functions.php file. If you’re not familiar with using code snippets, these articles have wonderful advice:
http://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/
http://www.wpbeginner.com/beginners-guide/beginners-guide-to-pasting-snippets-from-the-web-into-wordpress/