diff --git a/pyfacts/core.py b/pyfacts/core.py
index 0d881fe..63461f4 100644
--- a/pyfacts/core.py
+++ b/pyfacts/core.py
@@ -309,7 +309,7 @@ def _validate_frequency(
         provided_frequency = getattr(AllFrequencies, provided_frequency)
     start_date = data[0][0]
     end_date = data[-1][0]
-    overall_gap = (end_date - start_date).days
+    overall_gap = (end_date - start_date).days + 1
     num_data_points = len(data)
     # days_per_data = num_data_points / overall_gap