Updating exercise notebooks

Author: AllenDowney

code/chap12ex.ipynb

@@ -107,7 +107,7 @@
     "\n",
     "    daily[\"date\"] = daily.index\n",
     "    start = daily.date[0]\n",
-    "    one_year = np.timedelta64(1, \"Y\")\n",
+    "    one_year = np.timedelta64(365, \"D\")\n",
     "    daily[\"years\"] = (daily.date - start) / one_year\n",
     "\n",
     "    return daily"