Pat,
The long and the short of it is: Retention Expiry (right click on site, properties, expiry) can run in two modes. You can run it to go against the archive date (the date that the item was archived), or by it's mod date (usually the creation date of the message). Here's an example to help illustrate.
micah's got an email that is 30 days old. At the 31st day, he archives the email. The retention category (1 yr) is then applied. Expiry runs every evening at 11pm. If micah's using modify date, then on the 1yr, 32nd day, his email is deleted from the vault. If micah's using archive date, then on the 1yr, 61st day, his email is deleted from the vault.
m