diff --git a/dmapp/dmweb/get_period_times.py b/dmapp/dmweb/get_period_times.py
index 868d94f..554de03 100644
--- a/dmapp/dmweb/get_period_times.py
+++ b/dmapp/dmweb/get_period_times.py
@@ -276,10 +276,19 @@ def get_period_totals(start, end, task=None):
         return [{"ws": "No Data", "total": ""}]
 
     bfirst = aux_results[0]["before_first"]
+    start_delta = 0
 
     if bfirst:
         bfdate = bfirst["date"].replace(tzinfo=utctz)
-        start_delta = round((start - bfdate.astimezone(timezone)).total_seconds())
+        time_since_bfirst = round((start - bfdate.astimezone(timezone)).total_seconds())
+
+        # Only apply start_delta if the before_first switch actually crosses into the period
+        # If time_since_bfirst > bfirst["delta"], the switch ended before the period started
+        if time_since_bfirst <= bfirst["delta"]:
+            start_delta = time_since_bfirst
+            logger.debug(f"start_delta: {start_delta}s (bfirst crosses into period)")
+        else:
+            logger.debug(f"start_delta: 0s (bfirst ended before period start: {time_since_bfirst}s > {bfirst['delta']}s)")
 
     ldoc = aux_results[0]["last_doc"]
     lastdate = ldoc["date"].replace(tzinfo=utctz)