Static Caching gotcha
We've released the free Scheduled Cache Invalidator addon to help with this.
If you are using full static caching, future-posting entries does not work: when the sitemap is cached, it is cached until a change is made to an entry, taxonomy or is manually flushed.
In other words, if you create a post that won't appear until 'tomorrow', and the XML sitemap is generated, when tomorrow comes around and your new post becomes publicly visible, the XML sitemap won't be updated until the cache is invalidated.
You could achieve this by:
editing content
running a daily command that clears the sitemap cache
use the Scheduled Cache Invalidator addon - it's free, and made and supported by us
We suggest looking at the Scheduled Cache Invalidator addon - this gives you per-minute control on clearing portions of the cache that need to be cleared for that specific minute in time.