Fix a type error, add a type-check script command

This was preventing the build to vercel. I didn't see it on my command line. It would show up with build, but I added a type-check to jog my memory and to help with this, because we don't build here.

Author: kevinashworth

package.json

@@ -7,7 +7,8 @@
     "build": "next build",
     "start": "next start",
     "lint": "next lint",
-    "format": "prettier --write ."
+    "format": "prettier --write .",
+    "type-check": "tsc --noEmit"
   },
   "dependencies": {
     "@headlessui/react": "2.2.9",

src/app/sleep-data/chart-constants.ts

@@ -148,7 +148,7 @@ export const options: ApexOptions = {
     labels: {
       formatter: function (d) {
         const date = new Date(d);
-        return date.getDay() === 0 ? d.substring(5) : "";
+        return date.getDay() === 0 ? String(d).substring(5) : "";
       },
       style: {
         fontSize: "10px",