I'm a little lost.  All I need to do is setup a simple workflow that triggers an email notification to account owners when a contract is 120 days before expiration date.  I've figured out how to create an email template, and even how to create a workflow, but from what I can tell, workflows are only triggered when someone acts on a record (edits or creates).  I then got far enough to insert Time-based Triggers, which seemed like the answer to my problem, but the condition I want to apply "120 Days before 'Contract: End Date'" isn't available to me - apparently because End Date is a calculated field???

Any ideas?