diff --git a/db/migrations/014_bingo_updated_at.rb b/db/migrations/014_bingo_updated_at.rb
new file mode 100644
index 0000000..2df7048
--- /dev/null
+++ b/db/migrations/014_bingo_updated_at.rb
@@ -0,0 +1,13 @@
+Sequel.migration do
+  up do
+    # Last change to the row, for the "updated" date and sitemap lastmod.
+    %i[bingo_datacenters bingo_incidents].each do |t|
+      add_column t, :updated_at, DateTime
+      from(t).update(updated_at: :created_at)
+    end
+  end
+
+  down do
+    %i[bingo_datacenters bingo_incidents].each { |t| drop_column t, :updated_at }
+  end
+end
diff --git a/lib/bingo.rb b/lib/bingo.rb
index b1fb328..707e96c 100644
--- a/lib/bingo.rb
+++ b/lib/bingo.rb
@@ -99,6 +99,11 @@ module Bingo
       super
     end
 
+    def before_save
+      self.updated_at = Time.now.utc
+      super
+    end
+
     def operational? = status == 'operational'
     def planned?     = status == 'planned'
 
@@ -126,10 +131,21 @@ module Bingo
     include Localized
     many_to_one :datacenter, class: 'Bingo::Datacenter'
 
+    # A removed incident changes its datacenter's cell.
+    def after_destroy
+      super
+      Datacenter.where(id: datacenter_id).update(updated_at: Time.now.utc)
+    end
+
     def before_create
       self.created_at ||= Time.now.utc
       super
     end
+
+    def before_save
+      self.updated_at = Time.now.utc
+      super
+    end
   end
 
   # Board state for rendering: ordered cells, hits and completed lines.
@@ -145,7 +161,7 @@ module Bingo
       hit   = cells.map { |dc| dc.incidents.any? }
       wins  = LINES.select { |line| line.all? { |i| hit[i] } }
       incidents = cells.flat_map(&:incidents).sort_by { |i| [i.occurred_on, i.id] }.reverse
-      updated = [Datacenter.max(:created_at), Incident.max(:created_at)].compact.max
+      updated = [Datacenter.max(:updated_at), Incident.max(:updated_at)].compact.max
       new(cells, hit, wins, incidents, updated, view)
     end
 
@@ -364,7 +380,10 @@ module Bingo
   end
 
   def self.renumber!(ids = Datacenter.order(:position, :id).select_map(:id))
-    ids.each_with_index { |id, i| Datacenter.where(id: id).update(position: i + 1) }
+    now = Time.now.utc
+    ids.each_with_index do |id, i|
+      Datacenter.where(id: id).exclude(position: i + 1).update(position: i + 1, updated_at: now)
+    end
   end
 
   # "64" → 64.0, "25–30" → 30.0, ">40" → 40.0, "13,7" → 13.7, "" → nil
@@ -388,14 +407,14 @@ module Bingo
     path = File.join($root, 'db', 'bingo_seed.json')
     return unless File.exist?(path)
     seed = JSON.parse(File.read(path, encoding: 'UTF-8'))
-    cols = Datacenter.columns.map(&:to_s) - %w[id created_at]
+    cols = Datacenter.columns.map(&:to_s) - %w[id created_at updated_at]
     DB.connection.transaction do
       ids = seed.fetch('datacenters').each_with_index.to_h do |row, i|
         dc = Datacenter.create(row.slice(*cols).merge('position' => row['position'] || i + 1))
         [dc.slug, dc.id]
       end
       seed.fetch('incidents', []).each do |row|
-        Incident.create(row.slice(*(Incident.columns.map(&:to_s) - %w[id created_at datacenter_id]))
+        Incident.create(row.slice(*(Incident.columns.map(&:to_s) - %w[id created_at updated_at datacenter_id]))
                            .merge('datacenter_id' => ids.fetch(row.fetch('slug'))))
       end
     end
diff --git a/lib/bingo_admin.rb b/lib/bingo_admin.rb
index ac443a8..e010bbe 100644
--- a/lib/bingo_admin.rb
+++ b/lib/bingo_admin.rb
@@ -221,7 +221,7 @@ module BingoAdmin
 
     app.post '/admin/bingo/incidents/:id/delete' do
       inc = Bingo::Incident[params[:id].to_i]
-      Bingo::Incident.where(id: params[:id].to_i).delete
+      inc&.destroy # hook bumps the datacenter's updated_at
       audit!('bingo.incident_delete', inc&.datacenter&.slug, inc && "#{inc.occurred_on} #{inc.title_ru}")
       session[:bingo_flash] = 'Incident deleted.'
       redirect '/admin/bingo'
diff --git a/test/bingo_test.rb b/test/bingo_test.rb
index 9d6ab31..22a97e8 100644
--- a/test/bingo_test.rb
+++ b/test/bingo_test.rb
@@ -175,6 +175,21 @@ class BingoTest < Minitest::Test
     assert last_response.ok?
     %w[ru en uk].each { |l| assert_includes last_response.body, "<loc>https://example.org/#{l}</loc>" }
     assert_includes last_response.body, 'hreflang="x-default"'
+    refute_includes last_response.body, 'changefreq'
+    assert_match %r{</loc>\n    <lastmod>\d{4}-\d\d-\d\dT\d\d:\d\d:\d\dZ</lastmod>\n    <xhtml:link}, last_response.body
+  end
+
+  def test_updated_on_follows_edits_and_deletes
+    old = Time.utc(2026, 1, 1)
+    DB.connection[:bingo_datacenters].update(created_at: old, updated_at: old)
+    assert_equal old, Bingo::Board.load.updated_on
+    @dcs[5].refresh.update(name_en: 'Renamed')
+    assert_operator Bingo::Board.load.updated_on, :>, old
+    DB.connection[:bingo_datacenters].update(updated_at: old)
+    inc = hit!(2)
+    DB.connection[:bingo_incidents].update(created_at: old, updated_at: old)
+    inc.refresh.destroy
+    assert_operator Bingo::Board.load.updated_on, :>, old
   end
 
   def test_asxpio_static_files_not_served
diff --git a/views/bingo/sitemap.erb b/views/bingo/sitemap.erb
index 6267d6e..fe49fef 100644
--- a/views/bingo/sitemap.erb
+++ b/views/bingo/sitemap.erb
@@ -3,8 +3,8 @@
 <% Bingo::LOCALES.each do |l| %>
   <url>
     <loc><%= base_url %>/<%= l %></loc>
-<% if @lastmod %>    <lastmod><%= @lastmod.utc.strftime('%Y-%m-%d') %></lastmod>
-<% end %>    <changefreq>daily</changefreq>
+<% if @lastmod %>    <lastmod><%= @lastmod.utc.iso8601 %></lastmod>
+<% end %>
 <% Bingo::LOCALES.each do |alt| %>    <xhtml:link rel="alternate" hreflang="<%= alt %>" href="<%= base_url %>/<%= alt %>"/>
 <% end %>    <xhtml:link rel="alternate" hreflang="x-default" href="<%= base_url %>/"/>
   </url>