diff --git a/announce.sh b/announce.sh
index 5de1417..f10bd53 100755
--- a/announce.sh
+++ b/announce.sh
@@ -107,6 +107,14 @@ function get_tag_meta {
     echo "$TAG_META" | grep "^meta:$fieldname:" | cut -f2 -d' '
 }
 
+# How far back should we look for release info? If there is no
+# explicit metadata (signaled by passing "-"), use whatever previous
+# version number we were able to detect.
+DIFF_START=$(get_tag_meta diff-start)
+if [[ "$DIFF_START" == "-" ]]; then
+    DIFF_START="$PREVIOUS_VERSION"
+fi
+
 # The series name is part of the commit message left by release.sh.
 SERIES=$(get_tag_meta series)
 
@@ -148,7 +156,7 @@ if [[ "$INCLUDE_PYPI_LINK" == "yes" ]]; then
     include_pypi_link="--include-pypi-link"
 fi
 
-echo "$PREVIOUS_VERSION to $VERSION on $SERIES"
+echo "$DIFF_START to $VERSION on $SERIES"
 
 relnotes_file="$RELNOTESDIR/$SHORTNAME-$VERSION"
 
@@ -159,7 +167,7 @@ release-notes \
     --series $SERIES \
     $stable \
     $first_release \
-    . "$PREVIOUS_VERSION" "$VERSION" \
+    . "$DIFF_START" "$VERSION" \
     $include_pypi_link \
     | tee $relnotes_file
 
diff --git a/releasetools/cmds/propose_final_releases.py b/releasetools/cmds/propose_final_releases.py
index 7cdc068..b31e1b0 100644
--- a/releasetools/cmds/propose_final_releases.py
+++ b/releasetools/cmds/propose_final_releases.py
@@ -28,11 +28,19 @@ PROJECT_TEMPLATE = '''\
 
 VERSION_TEMPLATE = '''\
   - version: {version}
+    diff-start: {diff_start}
     projects:
 {projects}
 '''
 
 
+def get_prior_branch_point(version):
+    "Assuming we always branch on the rc1 tag..."
+    parts = version.split('.')
+    prior = int(parts[0]) - 1
+    return '{}.0.0.0rc1'.format(prior)
+
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument(
@@ -88,8 +96,10 @@ def main():
                 print('#  not a release candidate')
             continue
         new_version = latest_release['version'].split('.0rc')[0]
+        diff_start = get_prior_branch_point(new_version)
         deliverable_data['releases'].append({
             'version': new_version,
+            'diff_start': diff_start,
             'projects': latest_release['projects'],
         })
         print('new version for {}: {}'.format(os.path.basename(filename),
@@ -101,6 +111,7 @@ def main():
                              for p in latest_release['projects'])
         new_block = VERSION_TEMPLATE.format(
             version=new_version,
+            diff_start=diff_start,
             projects=projects,
         ).rstrip() + '\n'
         with open(filename, 'a') as f: