I was thinking about adding this but I don’t think it fits with the plugin. Allow me to explain my thinking:
With a fixed date, the process is either a one-off expiry or a per-post specific expiry date.
In the case of one-off expiry, the process is effectively a bulk delete which is easily achieved through the normal WP interface (all posts, sort by date, mark, trash).
If you’re looking at handling thousands of posts, a plugin like WP Mass Delete provides a quicker means of removing the posts, as well as many more options for filtering. Either way, a fixed date is not something that needs running on a schedule, as post dates rarely change.
In the case of individual date expiry (per post), it would require additional functionality such as adding a custom field with the expiry date, plus defaults and so on. Again, there are plugins that do this (like the excellent Post Expirator).
I wrote TIEexpire specifically because those plugins didn’t do what I wanted, which was the regular application of relative checks rather than absolutes. e.g. trash posts older than 30 days from whenever they were posted. Either that or they were too individual-post-specific or required too much effort to set dates on every single item.
This request is therefore outside of the remit of the plugin, so I won’t be adding it. I trust that makes sense. ??
