diff --git a/src/main.c b/src/main.c
index ccd2971..943775a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -175,6 +175,8 @@ void calc_data(Tel *t, Bat_state *b, long now) {
         }
 
         long dt = now - b->previous_ms;
+        if (dt < 0) dt = 0;
+
         b->previous_ms = now;
         b->wh_used += (float)t->power.data * (float)dt / 3600000.0f;
 }