diff --git a/lib/invoice_csv.rb b/lib/invoice_csv.rb
index f4e1bb4..b73997f 100644
--- a/lib/invoice_csv.rb
+++ b/lib/invoice_csv.rb
@@ -11,8 +11,14 @@ class InvoiceCsv
     CSV.generate do |csv|
       csv << HEADERS
       @invoices.each do |inv|
-        csv << [inv.number, inv.customer, inv.issued_on.to_s, format("%.2f", inv.total)]
+        csv << [inv.number, inv.customer, format_date(inv.issued_on), format("%.2f", inv.total)]
       end
     end
   end
+
+  private
+
+  def format_date(time, format: "%Y-%m-%d")
+    time.strftime(format)
+  end
 end
diff --git a/spec/invoice_csv_spec.rb b/spec/invoice_csv_spec.rb
index b6bb033..90f948b 100644
--- a/spec/invoice_csv_spec.rb
+++ b/spec/invoice_csv_spec.rb
@@ -7,4 +7,10 @@ RSpec.describe InvoiceCsv do
     csv = described_class.new([]).to_csv
     expect(csv.lines.first.chomp).to eq("number,customer,issued_on,total")
   end
+
+  it "writes issued_on as YYYY-MM-DD in the invoice's zone" do
+    issued = Time.new(2026, 3, 4, 23, 30, 0, "-08:00")
+    csv = described_class.new([Invoice.new("INV-7", "Acme", issued, 12.5)]).to_csv
+    expect(csv.lines.last.chomp).to eq("INV-7,Acme,2026-03-04,12.50")
+  end
 end