diff --git a/app/models/timer_session.rb b/app/models/timer_session.rb
index 8086d25a..201a706a 100644
--- a/app/models/timer_session.rb
+++ b/app/models/timer_session.rb
@@ -13,8 +13,8 @@ class TimerSession < RedmineTrackyApplicationRecord
   has_many :time_entries, through: :timer_session_time_entries
 
   validates :timer_start, presence: true
-  before_save :set_recorded_hours
   before_save :round_timer_to_nearest_minute
+  before_save :set_recorded_hours
 
   scope :active, -> { where(finished: false) }
   scope :finished, -> { where(finished: true) }
@@ -65,7 +65,7 @@ def can_overlap?(other_session)
   end
 
   def round_timer_to_nearest_minute
-    self.timer_start = round_to_nearest_minute(timer_start) if timer_end.present?
+    self.timer_start = round_to_nearest_minute(timer_start) if timer_start.present?
     self.timer_end = round_to_nearest_minute(timer_end) if timer_end.present?
   end
 
diff --git a/test/unit/timer_session_test.rb b/test/unit/timer_session_test.rb
index 1ecf7aaf..92cd2636 100644
--- a/test/unit/timer_session_test.rb
+++ b/test/unit/timer_session_test.rb
@@ -59,6 +59,7 @@ class TimerSessionTest < ActiveSupport::TestCase
   end
 
   test 'round_timer_to_nearest_minute' do
+    @timer_session.timer_end = nil
     timer_start_before = @timer_session.timer_start
 
     timer_start_after = timer_start_before + 10.seconds
@@ -106,4 +107,14 @@ class TimerSessionTest < ActiveSupport::TestCase
     assert session1.overlaps?(session2)
     assert session2.overlaps?(session1)
   end
+
+  test 'round_timer_start_when_timer_end_is_nil' do
+    timer_session = FactoryBot.build(:timer_session, user: User.current, finished: false)
+    timer_session.timer_end = nil
+    timer_session.timer_start = Time.zone.now + 45.seconds
+
+    timer_session.save!
+
+    assert_equal 0, timer_session.timer_start.sec, 'timer_start should be rounded to nearest minute even when timer_end is nil' # rubocop:disable Layout/LineLength
+  end
 end