add safeguard if clock does some undifined shenanigans

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;
 }