convert date range filter to client's timezone before filtering orders

app/views/admin/reports/_date_range_form.html.haml

@@ -8,3 +8,7 @@
     %span.range-divider
       %i.icon-arrow-right
     = f.text_field "#{field}_lt", :class => 'datetimepicker datepicker-to', :placeholder => t(:stop), data: { controller: "flatpickr", "flatpickr-enable-time-value": true, "flatpickr-default-date-value": "endOfDay" }
+= hidden_field_tag "time_zone"
+:javascript
+  document.querySelector("input[name='time_zone']").value = Intl.DateTimeFormat().resolvedOptions().timeZone
+

lib/reporting/reports/customers/base.rb

@@ -55,6 +55,10 @@ def filter_to_completed_at(orders)
 
           return orders if min.blank? || max.blank?
 
+          if client_time_zone.present?
+            min = convert_to_client_time_zone(min)
+            max = convert_to_client_time_zone(max)
+          end
           orders.where(completed_at: [min..max])
         end
 
@@ -81,6 +85,18 @@ def last_completed_order(orders)
         def last_completed_order_date(orders)
           last_completed_order(orders).completed_at&.to_date
         end
+
+        def convert_to_client_time_zone(datetime)
+          DateTime.parse(datetime).change(offset: utc_offset)
+        end
+
+        def client_time_zone
+          ActiveSupport::TimeZone[params[:time_zone] || ""]
+        end
+
+        def utc_offset
+          ActiveSupport::TimeZone[client_time_zone].formatted_offset
+        end
       end
     end
   end

spec/lib/reports/customers_report_spec.rb

@@ -277,6 +277,29 @@ module Customers
               allow(subject).to receive(:params).and_return(order_cycle_id: oc1.id)
               expect(subject.filter(orders)).to eq([order1])
             end
+
+            it "filters by date range using client's time zone" do
+              time_zone = ActiveSupport::TimeZone["Berlin"].formatted_offset
+              completed_at1 = DateTime.parse("2023/10/03 18:30")
+              completed_at2 = DateTime.parse("2023/10/03 19:30")
+
+              completed_at1 = completed_at1.change(offset: time_zone)
+              completed_at2 = completed_at2.change(offset: time_zone)
+
+              o1 = create(:order, completed_at: completed_at1)
+              o2 = create(:order, completed_at: completed_at2)
+
+              allow(subject).to receive(:params).and_return(
+                {
+                  q: {
+                    completed_at_gt: "2023/10/03 18:00",
+                    completed_at_lt: "2023/10/03 19:00"
+                  },
+                  time_zone: "Berlin"
+                }
+              )
+              expect(subject.filter(orders)).to eq([o1])
+            end
           end
         end
       end