Ævar Arnfjörð Bjarmason  [off-list ref] writes:

Move the declaration of the date.c functions from cache.h, and adjust
the relevant users to include the new date.h header.

It makes the patch larger than it could be to split off part of
cache.h into a new header and force users to include the new date.h
in the same commit, rather than first including date.h in cache.h
so that users do not have to change, and then update the inclusion
in a separate follow-up commit.   The end result looks OK, though.