diff --git a/modules/costs/config/locales/en.yml b/modules/costs/config/locales/en.yml
index cf4b1dc3dc40..bbbc33a33279 100644
--- a/modules/costs/config/locales/en.yml
+++ b/modules/costs/config/locales/en.yml
@@ -186,3 +186,8 @@ en:
 
   js:
     text_are_you_sure: "Are you sure?"
+
+  api_v3:
+    errors:
+      validation:
+        start_time_different_date: "Date part of startTime (%{start_time}) must be the same as the spentOn (%{spent_on}) date."
diff --git a/modules/costs/lib/api/v3/time_entries/schemas/time_entry_schema_representer.rb b/modules/costs/lib/api/v3/time_entries/schemas/time_entry_schema_representer.rb
index ef7efd5f5542..c5906a0e9d3e 100644
--- a/modules/costs/lib/api/v3/time_entries/schemas/time_entry_schema_representer.rb
+++ b/modules/costs/lib/api/v3/time_entries/schemas/time_entry_schema_representer.rb
@@ -50,6 +50,15 @@ def self.represented_class
           schema :spent_on,
                  type: "Date"
 
+          schema :start_time,
+                 type: "DateTime",
+                 required: -> { TimeEntry.must_track_start_and_end_time? },
+                 writable: -> { TimeEntry.can_track_start_and_end_time? }
+
+          schema :end_time,
+                 type: "DateTime",
+                 writable: false
+
           schema :hours,
                  type: "Duration"
 
diff --git a/modules/costs/lib/api/v3/time_entries/time_entry_representer.rb b/modules/costs/lib/api/v3/time_entries/time_entry_representer.rb
index ad87f115e7a3..6e85f0a0eb77 100644
--- a/modules/costs/lib/api/v3/time_entries/time_entry_representer.rb
+++ b/modules/costs/lib/api/v3/time_entries/time_entry_representer.rb
@@ -88,6 +88,16 @@ class TimeEntryRepresenter < ::API::Decorators::Single
                    datetime_formatter.format_duration_from_hours(represented.hours) if represented.hours
                  end
 
+        date_time_property :start_time,
+                           exec_context: :decorator,
+                           getter: ->(*) { datetime_formatter.format_datetime(represented.start_timestamp, allow_nil: true) },
+                           if: ->(*) { TimeEntry.can_track_start_and_end_time? }
+
+        date_time_property :end_time,
+                           exec_context: :decorator,
+                           getter: ->(*) { datetime_formatter.format_datetime(represented.end_timestamp, allow_nil: true) },
+                           if: ->(*) { TimeEntry.can_track_start_and_end_time? }
+
         date_time_property :created_at
         date_time_property :updated_at
 
@@ -129,6 +139,27 @@ def hours=(value)
                                                                          allow_nil: true)
         end
 
+        def start_time=(value) # rubocop:disable Metrics/AbcSize
+          ts = datetime_formatter.parse_datetime(value, "start_time", allow_nil: true)
+
+          if ts.nil?
+            represented.start_timestamp = nil
+            return
+          end
+
+          if represented.user.present?
+            ts = ts.in_time_zone(represented.user.time_zone)
+          end
+
+          if ts.to_date == represented.spent_on
+            represented.start_time = ts.strftime("%H:%M")
+          else
+            raise API::Errors::Validation.new("start_time",
+                                              I18n.t("api_v3.errors.validation.start_time_different_date",
+                                                     spent_on: represented.spent_on, start_time: ts.to_date))
+          end
+        end
+
         self.to_eager_load = [:work_package,
                               :user,
                               :activity,
diff --git a/modules/costs/lib/costs/patches/permitted_params_patch.rb b/modules/costs/lib/costs/patches/permitted_params_patch.rb
index af4c1bcdfcc8..c3d4d269c683 100644
--- a/modules/costs/lib/costs/patches/permitted_params_patch.rb
+++ b/modules/costs/lib/costs/patches/permitted_params_patch.rb
@@ -66,13 +66,17 @@ def user_rates
     end
 
     def time_entries
+      additional_fields = []
+
+      additional_fields << :start_time if TimeEntry.can_track_start_and_end_time?
+
       params
         .require(:time_entry)
         .permit(
+          *additional_fields,
           :hours,
           :comments,
           :spent_on,
-          :start_time,
           :work_package_id,
           :activity_id,
           :project_id,
diff --git a/modules/costs/spec/factories/time_entry_factory.rb b/modules/costs/spec/factories/time_entry_factory.rb
index 325985cff303..43c1aac95b7b 100644
--- a/modules/costs/spec/factories/time_entry_factory.rb
+++ b/modules/costs/spec/factories/time_entry_factory.rb
@@ -38,5 +38,11 @@
     after(:build) do |time_entry|
       time_entry.project ||= time_entry.work_package.project
     end
+
+    trait :with_start_and_end_time do
+      time_zone { "Asia/Tokyo" }
+      start_time { 390 } # 6:30 AM
+      hours { 2.5 }
+    end
   end
 end
diff --git a/modules/costs/spec/lib/api/v3/time_entries/schemas/time_entry_schema_representer_spec.rb b/modules/costs/spec/lib/api/v3/time_entries/schemas/time_entry_schema_representer_spec.rb
index 3eee3e2c1383..c30734b03dff 100644
--- a/modules/costs/spec/lib/api/v3/time_entries/schemas/time_entry_schema_representer_spec.rb
+++ b/modules/costs/spec/lib/api/v3/time_entries/schemas/time_entry_schema_representer_spec.rb
@@ -108,6 +108,53 @@
       end
     end
 
+    describe "startTime" do
+      let(:path) { "startTime" }
+
+      before do
+        allow(TimeEntry).to receive_messages(
+          can_track_start_and_end_time?: can_track_times,
+          must_track_start_and_end_time?: must_track_times
+        )
+      end
+
+      context "when start- and end-time tracking is disabled" do
+        let(:can_track_times) { false }
+        let(:must_track_times) { false }
+
+        it_behaves_like "has basic schema properties" do
+          let(:type) { "DateTime" }
+          let(:name) { TimeEntry.human_attribute_name("start_time") }
+          let(:required) { false }
+          let(:writable) { false }
+        end
+      end
+
+      context "when start- and end-time tracking is enabled" do
+        let(:can_track_times) { true }
+        let(:must_track_times) { false }
+
+        it_behaves_like "has basic schema properties" do
+          let(:type) { "DateTime" }
+          let(:name) { TimeEntry.human_attribute_name("start_time") }
+          let(:required) { false }
+          let(:writable) { true }
+        end
+      end
+
+      context "when start- and end-time tracking is enforced" do
+        let(:can_track_times) { true }
+        let(:must_track_times) { true }
+
+        it_behaves_like "has basic schema properties" do
+          let(:type) { "DateTime" }
+          let(:name) { TimeEntry.human_attribute_name("start_time") }
+          let(:required) { true }
+          let(:writable) { true }
+        end
+      end
+    end
+
     describe "hours" do
       let(:path) { "hours" }
 
diff --git a/modules/costs/spec/lib/api/v3/time_entries/time_entry_representer_parsing_spec.rb b/modules/costs/spec/lib/api/v3/time_entries/time_entry_representer_parsing_spec.rb
index 0c7afd3b3dee..72b5c0bc57b2 100644
--- a/modules/costs/spec/lib/api/v3/time_entries/time_entry_representer_parsing_spec.rb
+++ b/modules/costs/spec/lib/api/v3/time_entries/time_entry_representer_parsing_spec.rb
@@ -81,6 +81,7 @@
         "raw" => "some comment"
       },
       "spentOn" => "2017-07-28",
+      "startTime" => "2017-07-28T12:30:00Z",
       text_custom_field.attribute_name(:camel_case) => {
         "raw" => "some text"
       }
@@ -129,7 +130,7 @@
   end
 
   describe "properties" do
-    context "spentOn" do
+    describe "spentOn" do
       it "updates spent_on" do
         time_entry = representer.from_hash(hash)
         expect(time_entry.spent_on)
@@ -137,7 +138,62 @@
       end
     end
 
-    context "hours" do
+    describe "startTime" do
+      context "when not tracking start and end time" do
+        before do
+          allow(TimeEntry).to receive_messages(
+            can_track_start_and_end_time?: false,
+            must_track_start_and_end_time?: false
+          )
+        end
+
+        it "does not set start_time" do
+          time_entry = representer.from_hash(hash)
+          expect(time_entry.start_time).to be_nil
+        end
+      end
+
+      context "when tracking start and end time" do
+        before do
+          allow(TimeEntry).to receive_messages(
+            can_track_start_and_end_time?: true,
+            must_track_start_and_end_time?: false
+          )
+        end
+
+        context "when spent_on != start_time date" do
+          before do
+            hash["startTime"] = "1980-12-22T12:00:00Z"
+          end
+
+          it "raises an error" do
+            expect do
+              representer.from_hash(hash)
+            end.to raise_error(API::Errors::Validation)
+          end
+        end
+
+        it "sets start_time" do
+          user.pref[:time_zone] = "Asia/Tokyo"
+
+          time_entry = representer.from_hash(hash)
+
+          # timezone on the TimeEntry would be set to the user's TimeZone via the SetAttribute service, so we need to
+          # manually set it here
+          time_entry.time_zone = "Asia/Tokyo"
+
+          # We are sending in 12:30:00 UTC as the start time, in Tokyo time (for 2017-07-28) that equals
+          # 21:30:00 in Japan Standard Time (JST), so the time should be set to 21:30
+
+          expect(time_entry.start_time).to eq((21 * 60) + 30) # 12:30
+
+          expect(time_entry.start_timestamp).to eq(DateTime.parse("2017-07-28T12:30:00Z"))
+          expect(time_entry.end_timestamp).to eq(DateTime.parse("2017-07-28T17:30:00Z"))
+        end
+      end
+    end
+
+    describe "hours" do
       it "updates hours" do
         time_entry = representer.from_hash(hash)
         expect(time_entry.hours)
@@ -159,7 +215,7 @@
       end
     end
 
-    context "comment" do
+    describe "comment" do
       it "updates comment" do
         time_entry = representer.from_hash(hash)
         expect(time_entry.comments)
@@ -167,7 +223,7 @@
       end
     end
 
-    context "property custom field" do
+    describe "property custom field" do
       it "updates the custom value" do
         time_entry = representer.from_hash(hash)
 
diff --git a/modules/costs/spec/requests/api/time_entry_resource_spec.rb b/modules/costs/spec/requests/api/time_entry_resource_spec.rb
index 1117df243b15..1cb7b195a260 100644
--- a/modules/costs/spec/requests/api/time_entry_resource_spec.rb
+++ b/modules/costs/spec/requests/api/time_entry_resource_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #-- copyright
 # OpenProject is an open source project management software.
 # Copyright (C) the OpenProject GmbH
@@ -330,6 +332,76 @@
           .at_path("errorIdentifier")
       end
     end
+
+    context "when start- & end-time tracking is enabled",
+            with_flag: { track_start_and_end_times_for_time_entries: true },
+            with_settings: { allow_tracking_start_and_end_times: true } do
+      context "when start and end time were tracked" do
+        let!(:time_entry) do
+          create(:time_entry, :with_start_and_end_time, project:, work_package:, user: current_user)
+        end
+
+        it "includes start and end timestamps in the API response" do
+          get api_v3_paths.time_entries
+
+          expect(subject.body)
+            .to be_json_eql(time_entry.start_timestamp.utc.to_json)
+            .at_path("_embedded/elements/0/startTime")
+
+          expect(subject.body)
+            .to be_json_eql(time_entry.end_timestamp.utc.to_json)
+            .at_path("_embedded/elements/0/endTime")
+        end
+      end
+
+      context "when start and end time were not tracked" do
+        let!(:time_entry) do
+          create(:time_entry, project:, work_package:, user: current_user)
+        end
+
+        it "returns nil as start and end timestamps in the API response" do
+          get api_v3_paths.time_entries
+
+          expect(subject.body)
+            .to be_json_eql(nil.to_json)
+            .at_path("_embedded/elements/0/startTime")
+
+          expect(subject.body)
+            .to be_json_eql(nil.to_json)
+            .at_path("_embedded/elements/0/endTime")
+        end
+      end
+    end
+
+    context "when start- & end-time tracking is disabled",
+            with_flag: { track_start_and_end_times_for_time_entries: false },
+            with_settings: { allow_tracking_start_and_end_times: false } do
+      context "when start and end time were tracked" do
+        let!(:time_entry) do
+          create(:time_entry, :with_start_and_end_time, project:, work_package:, user: current_user)
+        end
+
+        it "does not include start and end time fields" do
+          get api_v3_paths.time_entries
+
+          expect(subject.body).not_to have_json_path("_embedded/elements/0/startTime")
+          expect(subject.body).not_to have_json_path("_embedded/elements/0/endTime")
+        end
+      end
+
+      context "when start and end time were not tracked" do
+        let!(:time_entry) do
+          create(:time_entry, project:, work_package:, user: current_user)
+        end
+
+        it "does not include start and end time fields" do
+          get api_v3_paths.time_entries
+
+          expect(subject.body).not_to have_json_path("_embedded/elements/0/startTime")
+          expect(subject.body).not_to have_json_path("_embedded/elements/0/endTime")
+        end
+      end
+    end
   end
 
   describe "GET /api/v3/time_entries/:id" do
@@ -617,7 +689,7 @@
       end
 
       it "removes the time_entry from the DB" do
-        expect(TimeEntry.exists?(time_entry.id)).to be_falsey
+        expect(TimeEntry).not_to exist(time_entry.id)
       end
     end
 
@@ -627,7 +699,7 @@
       end
 
       it "does not delete the time_entry" do
-        expect(TimeEntry.exists?(time_entry.id)).to be_truthy
+        expect(TimeEntry).to exist(time_entry.id)
       end
     end