Earlier versions of a theme guide, by date
Turn on versions and a guide keeps how it read at the end of each day a run changed it. Links always open the latest; a date picker opens an earlier day, with what that day changed.
A guide keeps one version a day, for the last 30 days it changed. Each has its own address under /on/<date>, so a link to how a guide read on a given day keeps working.
Pages a day did not change are stored once, and their pictures are kept for as long as a version shows them.
More in Theme documentation.