diff --git a/tests/test_tracer.py b/tests/test_tracer.py
index 84e27c8..421b119 100644
--- a/tests/test_tracer.py
+++ b/tests/test_tracer.py
@@ -19,17 +19,18 @@ class TestRules(unittest.TestCase):
 	def setUp(self):
 		Applications._apps = ApplicationsCollection()
 		self.tracer = Tracer(PackageManagerMock(), Rules, Applications, memory=dump_memory_mock)
-		self.tracer.timestamp = 5555  # Sure, it should be a UNIX timestamp value
 		Applications._append_application({"name": "kernel", "ignore": True})
 		Application.processes_factory = ProcessesMock
 
 	@patch('tracer.resources.applications.System.init_system', return_value="dummy")
-	def test_trace_affected(self, init_system):
+	@patch('tracer.resources.applications.System.boot_time', return_value=5555)
+	def test_trace_affected(self, boot_time, init_system):
 		affected = self.tracer.trace_affected()
 		self.assertSetEqual(set(affected), set([Applications.find("baz"), Applications.find("qux")]))
 		self.assertIsInstance(affected, ApplicationsCollection)
 
-	def test_trace_application(self):
+	@patch('tracer.resources.applications.System.boot_time', return_value=5555)
+	def test_trace_application(self, boot_time):
 		affected = self.tracer.trace_application(Applications.find("baz"), AffectedProcessMock)
 		self.assertIsInstance(affected, AffectedProcessesCollection)
 		self.assertEqual(len(affected), 1)
diff --git a/tracer/resources/args_parser.py b/tracer/resources/args_parser.py
index 4eef578..243bfbb 100644
--- a/tracer/resources/args_parser.py
+++ b/tracer/resources/args_parser.py
@@ -58,6 +58,7 @@
 parser.add_argument('-t', '--timestamp',
 	nargs=1,
 	default=[None],
+	type=float,
 	dest='timestamp',
 	help='since when the updates should be'
 )
diff --git a/tracer/resources/tracer.py b/tracer/resources/tracer.py
index c2c19df..5d0d3e8 100644
--- a/tracer/resources/tracer.py
+++ b/tracer/resources/tracer.py
@@ -79,11 +79,19 @@ def _modified_packages(self):
 		if self.specified_packages and self.now:
 			return PackagesCollection(self.specified_packages)
 
-		timestamp = self.timestamp if self.timestamp else System.boot_time()
+		timestamp = self._timestamp()
 		packages = self._PACKAGE_MANAGER.packages_newer_than(timestamp)
 		packages = packages.intersection(self.specified_packages)
 		return packages
 
+	def _timestamp(self):
+		boot = System.boot_time()
+		if not self.timestamp:
+			return boot
+		if self.timestamp < boot:
+			return boot
+		return self.timestamp
+
 	def trace_affected(self, user=None):
 		"""
 		Returns collection of applications which uses some files that have been modified