help with data base.

I have found plugins to run cron jobs and I can create a cron job. But I need help with this. I want to run a cronjob to delete blod entrys over 60 days old. I can create the cron job . But I have no ide how to write the sql query.  Can any one help with this please. PS I can write the php to run the query as well . I just need help with the query string. I dont understand the elgg database well enough to know wher the data is stored.