Avoid the total time spent to have a negative value.

app/models/concerns/time_trackable.rb

@@ -15,7 +15,7 @@ module TimeTrackable
   end
 
   def spend_time=(seconds)
-    return unless seconds
+    return if invalid_time_spent?(seconds)
 
     new_time_spent = seconds.zero? ? -(total_time_spent) : seconds
     timelogs.new(time_spent: new_time_spent)
@@ -26,4 +26,14 @@ module TimeTrackable
   def total_time_spent
     timelogs.sum(:time_spent)
   end
+
+  private
+
+  def invalid_time_spent?(seconds)
+    return true unless seconds
+    # time to subtract exceeds the total time spent
+    return true if seconds < 0 && (seconds.abs > total_time_spent)
+
+    false
+  end
 end

spec/models/concerns/issuable_spec.rb

@@ -384,4 +384,36 @@ describe Issue, "Issuable" do
       expect(issue.assignee_or_author?(user)).to eq(false)
     end
   end
+
+  describe '#spend_time' do
+    let(:issue) { create(:issue) }
+
+    context 'adding time' do
+      it 'should update the total time spent' do
+        issue.update_attributes!(spend_time: 1800)
+
+        expect(issue.total_time_spent).to eq(1800)
+      end
+    end
+
+    context 'substracting time' do
+      before do
+        issue.update_attributes!(spend_time: 1800)
+      end
+
+      it 'should update the total time spent' do
+        issue.update_attributes!(spend_time: -900)
+
+        expect(issue.total_time_spent).to eq(900)
+      end
+
+      context 'when time to substract exceeds the total time spent' do
+        it 'should not alter the total time spent' do
+          issue.update_attributes!(spend_time: -3600)
+
+          expect(issue.total_time_spent).to eq(1800)
+        end
+      end
+    end
+  end
 end

spec/services/slash_commands/interpret_service_spec.rb

@@ -226,6 +226,14 @@ describe SlashCommands::InterpretService, services: true do
       end
     end
 
+    shared_examples 'spend command with negative time' do
+      it 'populates spend_time: "-1800" if content contains /spend -30m' do
+        _, updates = service.execute(content, issuable)
+
+        expect(updates).to eq(spend_time: -1800)
+      end
+    end
+
     shared_examples 'remove_estimation command' do
       it 'populates time_estimate: "0" if content contains /remove_estimation' do
         _, updates = service.execute(content, issuable)
@@ -503,6 +511,11 @@ describe SlashCommands::InterpretService, services: true do
       let(:issuable) { issue }
     end
 
+    it_behaves_like 'spend command with negative time' do
+      let(:content) { '/spend -30m' }
+      let(:issuable) { issue }
+    end
+
     it_behaves_like 'empty command' do
       let(:content) { '/spend' }
       let(:issuable) { issue }