diff --git a/homeassistant/components/time_date/sensor.py b/homeassistant/components/time_date/sensor.py
index 8be0e2d8c66..86ff3531240 100644
--- a/homeassistant/components/time_date/sensor.py
+++ b/homeassistant/components/time_date/sensor.py
@@ -116,16 +116,6 @@ class TimeDateSensor(Entity):
         date = dt_util.as_local(time_date).date().isoformat()
         date_utc = time_date.date().isoformat()
 
-        # Calculate Swatch Internet Time.
-        time_bmt = time_date + timedelta(hours=1)
-        delta = timedelta(
-            hours=time_bmt.hour,
-            minutes=time_bmt.minute,
-            seconds=time_bmt.second,
-            microseconds=time_bmt.microsecond,
-        )
-        beat = int((delta.seconds + delta.microseconds / 1000000.0) / 86.4)
-
         if self.type == "time":
             self._state = time
         elif self.type == "date":
@@ -139,6 +129,19 @@ class TimeDateSensor(Entity):
         elif self.type == "time_utc":
             self._state = time_utc
         elif self.type == "beat":
+            # Calculate Swatch Internet Time.
+            time_bmt = time_date + timedelta(hours=1)
+            delta = timedelta(
+                hours=time_bmt.hour,
+                minutes=time_bmt.minute,
+                seconds=time_bmt.second,
+                microseconds=time_bmt.microsecond,
+            )
+
+            # Use integers to better handle rounding. For example,
+            # int(63763.2/86.4) = 737 but 637632//864 = 738.
+            beat = int(delta.total_seconds() * 10) // 864
+
             self._state = f"@{beat:03d}"
         elif self.type == "date_time_iso":
             self._state = dt_util.parse_datetime(f"{date} {time}").isoformat()
diff --git a/tests/components/time_date/test_sensor.py b/tests/components/time_date/test_sensor.py
index b5116478678..bf55ea8075b 100644
--- a/tests/components/time_date/test_sensor.py
+++ b/tests/components/time_date/test_sensor.py
@@ -69,6 +69,8 @@ async def test_states(hass):
     device = time_date.TimeDateSensor(hass, "beat")
     device._update_internal_state(now)
     assert device.state == "@079"
+    device._update_internal_state(dt_util.utc_from_timestamp(1602952963.2))
+    assert device.state == "@738"
 
     device = time_date.TimeDateSensor(hass, "date_time_iso")
     device._update_internal_state(now)