diff --git a/hr_timesheet_begin_end/models/account_analytic_line.py b/hr_timesheet_begin_end/models/account_analytic_line.py
index 40bbbeac0..70a6e4dcf 100644
--- a/hr_timesheet_begin_end/models/account_analytic_line.py
+++ b/hr_timesheet_begin_end/models/account_analytic_line.py
@@ -16,9 +16,29 @@ class AccountAnalyticLine(models.Model):
     time_start = fields.Float(string="Begin Hour")
     time_stop = fields.Float(string="End Hour")
 
-    @api.onchange("time_start", "time_stop", "project_id")
-    def onchange_hours_start_stop(self):
-        self.unit_amount = self.unit_amount_from_start_stop()
+    # Override to be a computed field.
+    unit_amount = fields.Float(
+        compute="_compute_unit_amount",
+        store=True,
+        readonly=False,
+        # This default is a workaround for a bizarre situation: if a line is
+        # created with a time range but WITHOUT defining unit_amount, then you
+        # would expect unit_amount to be computed from the range. But this never
+        # happens, and it is instead set to default value 0. Subsequently the
+        # constraint _validate_unit_amount_equal_to_time_diff kicks in and
+        # raises an exception.
+        #
+        # By setting the default to None, the computation is correctly
+        # triggered. If nothing is computed, None falls back to 0.
+        default=None,
+    )
+
+    @api.depends("time_start", "time_stop", "project_id")
+    def _compute_unit_amount(self):
+        # Do not compute/adjust the unit_amount of non-timesheets.
+        lines = self.filtered(lambda line: line.project_id)
+        for line in lines:
+            line.unit_amount = line.unit_amount_from_start_stop()
 
     def _validate_start_before_stop(self):
         value_to_html = self.env["ir.qweb.field.float_time"].value_to_html
diff --git a/hr_timesheet_begin_end/readme/newsfragments/692.feature.2.rst b/hr_timesheet_begin_end/readme/newsfragments/692.feature.2.rst
new file mode 100644
index 000000000..185296c90
--- /dev/null
+++ b/hr_timesheet_begin_end/readme/newsfragments/692.feature.2.rst
@@ -0,0 +1 @@
+Changed ``unit_amount`` into a computed (stored, writeable) field.
diff --git a/hr_timesheet_begin_end/tests/test_timesheet_begin_end.py b/hr_timesheet_begin_end/tests/test_timesheet_begin_end.py
index 85ee99d01..7992565ae 100644
--- a/hr_timesheet_begin_end/tests/test_timesheet_begin_end.py
+++ b/hr_timesheet_begin_end/tests/test_timesheet_begin_end.py
@@ -21,18 +21,39 @@ def setUp(self):
             "employee_id": self.employee.id,
         }
 
-    def test_onchange(self):
-        line = self.timesheet_line_model.new(
-            {"name": "test", "time_start": 10.0, "time_stop": 12.0}
-        )
-        line.onchange_hours_start_stop()
-        self.assertEqual(line.unit_amount, 2)
+    def test_compute_unit_amount(self):
+        line = self.base_line.copy()
+        del line["unit_amount"]
+        line_record = self.timesheet_line_model.create(line)
+        self.assertEqual(line_record.unit_amount, 2)
+        line_record.time_stop = 14.0
+        self.assertEqual(line_record.unit_amount, 4)
+
+    def test_compute_unit_amount_no_compute_if_no_times(self):
+        line = self.base_line.copy()
+        del line["time_start"]
+        del line["time_stop"]
+        line_record = self.timesheet_line_model.create(line)
+        self.assertEqual(line_record.unit_amount, 2.0)
+        line_record.unit_amount = 3.0
+        self.assertEqual(line_record.unit_amount, 3.0)
+
+    def test_compute_unit_amount_to_zero(self):
+        line = self.base_line.copy()
+        del line["unit_amount"]
+        line_record = self.timesheet_line_model.create(line)
+        self.assertEqual(line_record.unit_amount, 2)
+        line_record.write({"time_start": 0, "time_stop": 0})
+        self.assertEqual(line_record.unit_amount, 0)
 
-    def test_onchange_no_update(self):
+    def test_compute_unit_amount_to_zero_no_record(self):
+        # Cannot create/save this model because it breaks a constraint, so using
+        # .new().
         line = self.timesheet_line_model.new(
             {"name": "test", "time_start": 13.0, "time_stop": 12.0}
         )
-        line.onchange_hours_start_stop()
+        self.assertEqual(line.unit_amount, 0)
+        line.time_stop = 10.0
         self.assertEqual(line.unit_amount, 0)
 
     def test_check_begin_before_end(self):