diff --git a/canopen/timestamp.py b/canopen/timestamp.py
index 21dcc636..3ef306a8 100644
--- a/canopen/timestamp.py
+++ b/canopen/timestamp.py
@@ -26,7 +26,7 @@ def transmit(self, timestamp: Optional[float] = None):
         :param float timestamp:
             Optional Unix timestamp to use, otherwise the current time is used.
         """
-        delta = timestamp or time.time() - OFFSET
+        delta = (timestamp or time.time()) - OFFSET
         days, seconds = divmod(delta, ONE_DAY)
         data = TIME_OF_DAY_STRUCT.pack(int(seconds * 1000), int(days))
         self.network.send_message(self.cob_id, data)
diff --git a/test/test_time.py b/test/test_time.py
index acd2b490..8a4e7095 100644
--- a/test/test_time.py
+++ b/test/test_time.py
@@ -1,6 +1,10 @@
 import unittest
+from unittest.mock import patch
+import time
+from datetime import datetime
 
 import canopen
+import canopen.timestamp
 
 
 class TestTime(unittest.TestCase):
@@ -10,13 +14,36 @@ def test_time_producer(self):
         network.NOTIFIER_SHUTDOWN_TIMEOUT = 0.0
         network.connect(interface="virtual", receive_own_messages=True)
         producer = canopen.timestamp.TimeProducer(network)
-        producer.transmit(1486236238)
-        msg = network.bus.recv(1)
-        network.disconnect()
-        self.assertEqual(msg.arbitration_id, 0x100)
-        self.assertEqual(msg.dlc, 6)
-        self.assertEqual(msg.data, b"\xb0\xa4\x29\x04\x31\x43")
 
+        # Test that the epoch is correct
+        epoch = datetime.strptime("1984-01-01 00:00:00 +0000", "%Y-%m-%d %H:%M:%S %z").timestamp()
+        self.assertEqual(int(epoch), canopen.timestamp.OFFSET)
+
+        current = time.time()
+        with patch("canopen.timestamp.time.time", return_value=current):
+            current_in_epoch = current - epoch
+
+            # Test it looking up the current time
+            producer.transmit()
+            msg = network.bus.recv(1)
+            self.assertEqual(msg.arbitration_id, 0x100)
+            self.assertEqual(msg.dlc, 6)
+            ms, days = canopen.timestamp.TIME_OF_DAY_STRUCT.unpack(msg.data)
+            self.assertEqual(days, int(current_in_epoch) // canopen.timestamp.ONE_DAY)
+            self.assertEqual(ms, int((current_in_epoch % canopen.timestamp.ONE_DAY) * 1000))
+
+            # Test providing a timestamp
+            faketime = 1_927_999_438  # 2031-02-04 20:23:58
+            producer.transmit(faketime)
+            msg = network.bus.recv(1)
+            self.assertEqual(msg.arbitration_id, 0x100)
+            self.assertEqual(msg.dlc, 6)
+            ms, days = canopen.timestamp.TIME_OF_DAY_STRUCT.unpack(msg.data)
+            current_in_epoch = faketime - epoch
+            self.assertEqual(days, int(current_in_epoch) // canopen.timestamp.ONE_DAY)
+            self.assertEqual(ms, int((current_in_epoch % canopen.timestamp.ONE_DAY) * 1000))
+
+        network.disconnect()
 
 if __name__ == "__main__":
     unittest.main()