Tweak the date used for snapshot.d.o references

Use *tomorrow* rather than today, to allow time for the snapshot to be
updated.

Makefile

@@ -166,7 +166,7 @@ endif
 endif
 $(BDIR)/DATE:
 	$(Q)date -u '+%Y%m%d' > $(BDIR)/DATE
-	$(Q)date -u '+%Y%m%dT%H%M%SZ' > $(BDIR)/DATE-zulu
+	$(Q)date -u '+%Y%m%dT%H%M%SZ' --date tomorrow > $(BDIR)/DATE-zulu
 
 ifdef MIRROR
 LATEST_DB := $(MIRROR)/$(shell $(which_deb) $(MIRROR) $(CODENAME) debootstrap)

debian/changelog

@@ -9,6 +9,8 @@ debian-cd (3.1.21-stretch) UNRELEASED; urgency=medium
     + Make tools/update_popcon deal with https on popcon.d.o
     + Ensure that in all places where we use -J we also use -joliet-long
     + List jigdo, template and torrent files in SUMS files too
+    + Tweak the date used for snapshot.d.o references - use *tomorrow*
+      rather than today, to allow time for the snapshot to be updated.
 
  -- Steve McIntyre <93sam@debian.org>  Mon, 19 Jun 2017 13:39:58 +0100