The idea is fairly straightforward: adjust the dates from a Blueprint course to the dates of a new shell as part of the sync process.
Here's why (I'm pretty sure it's obvious, though):
Blueprint shells are, in essence, "Master" shells that can be copied over and over with restrictions put in place to prevent unwanted tampering of the material. However, dates used in a Blueprint shell likely reflect a single time period/term, but the shell may be used for multiple time periods/terms. When syncing a new shell to the Blueprint, though, the dates of the Blueprint are used, instead of being adjusted (like they are when copying a course).