Fix day unit calculation so N=1 means yesterday, consistent with M/Y logic

Previously N=1 subtracted 1 day from yesterday (giving the day before yesterday),
which was inconsistent with month/year where N=1 means the current period.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: boxplotinc

Incrementally Refresh Dataflow.ipynb

@@ -911,9 +911,9 @@
     "            return None\n",
     "\n",
     "        if unit == 'D':\n",
-    "            # Days: subtract N days from yesterday\n",
-    "            result = yesterday - timedelta(days=n)\n",
-    "            self.logger.info(f\"Calculated start date: {result} (yesterday - {n} days)\")\n",
+    "            # Days: subtract (N-1) days from yesterday so N=1 means yesterday,\n            # consistent with month/year logic where N=1 means the current period.\n",
+    "            result = yesterday - timedelta(days=n - 1)\n",
+    "            self.logger.info(f\"Calculated start date: {result} (yesterday - {n - 1} days)\")\n",
     "        elif unit == 'M':\n",
     "            # Months: start of (current month - (N-1) months)\n",
     "            # N=1 means current month, N=2 means previous month, etc.\n",