How can I add a time period to a date field that will trigger a workflow

I'm trying to use a date field in the user table called "Hire Date" and create a workflow that looks at that date and sends an email to HR every 6 months reminding them they need to review this person.  I can't figure out a way to do this.  I know how to create the e-mail and trigger other events, but not how to add time to a date field before the workflow kicks in.

Thanks,