feat: Show the build date on the loading banner of unreleased builds (#2659)

scripts/build-date-plugin.js

@@ -0,0 +1,40 @@
+/**
+ * Vite plugin: writes the date of the build into the card.
+ */
+
+// Strict length requirement: See below.
+export const BUILD_DATE_PLACEHOLDER = '__BUILD_DATE_SENTINEL___';
+
+/**
+ * The date cannot be handed to the bundler as a substituted name, because the
+ * build configuration is evaluated once and `vite build --watch` reuses it.
+ * As such, every rebuild would report the time the watcher started rather than
+ * the actual build time. Output is generated afresh for each build, so the date
+ * is written here instead.
+ *
+ * @type {() => import('vite').Plugin}
+ */
+export const buildDate = () => ({
+  name: 'build-date',
+
+  renderChunk(code) {
+    if (!code.includes(BUILD_DATE_PLACEHOLDER)) {
+      return null;
+    }
+
+    const date = new Date().toISOString();
+    if (date.length !== BUILD_DATE_PLACEHOLDER.length) {
+      this.error(
+        'The build date must be exactly as long as the placeholder it replaces.',
+      );
+    }
+
+    return {
+      code: code.replaceAll(BUILD_DATE_PLACEHOLDER, date),
+
+      // The replacement is exactly as long as what it replaces, so the existing
+      // sourcemap still describes the code.
+      map: null,
+    };
+  },
+});