It is set_site_transient
, not set_site_site_transient
.
You should be aware that a single user will still wait that half a minute for this to complete. wp_cron
does not run on clock time. It runs when a user visits the site.
If you have the ability to create a system cron
job then hook your callback to an AJAX API action, and use the system cron
to trigger it. That should give you a true clock-time activation and avoid having to force some unlucky user to wait.
Related:
WordPress Cron Schedule the if and else statement
Should I use set_transient or update_option?