设置服务以在Web集群环境中执行任务

I am looking for ways to reliably execute a task (just one time at certain date/time) in web clustered environment: for example to send an email reminder to user after 7 days since initial registration. I am aware of ways to accomplish that using cron/django-cron/django-chronograph etc, but everything I found so far seem to be designed for single-server scenario. I am looking for ways to reliably execute a tas