[gh-pages] Compute the specs' Date metadata from the git timestamps

Author: andreubotella

.github/workflows/build-specs.yml

@@ -45,7 +45,13 @@ jobs:
         run: |
           set -e
           for file in ./**/*.bs; do
-            bikeshed -f spec "$file"
+            # We use `date` to build a YYYY-MM-DD date rather than using git's
+            # `--format=%as` because we want the date not to depend on the
+            # committer's timezone. We use UTC to avoid depending on the build
+            # runner's timezone as well.
+            TIMESTAMP="$(git log -1 --format=%at "$file")"
+            SHORT_DATE="$(date --date=@"$TIMESTAMP" --utc +%F)"
+            bikeshed -f --md-date="$SHORT_DATE" spec "$file"
           done
       - name: Rename to index.html
         # The default pages for directories are assumed to be Overview.html, but