diff --git a/app/models/person.rb b/app/models/person.rb
index 66462d8..cbc3c8f 100644
--- a/app/models/person.rb
+++ b/app/models/person.rb
@@ -11,6 +11,8 @@ class Person < ApplicationRecord
           through:    :account,
           source:     :own_membership_app
 
+  validate :supporter_since_not_in_future
+
   def related_to_party?
     party_supporter? || party_member? || excluded_from_party?
   end
@@ -26,4 +28,12 @@ class Person < ApplicationRecord
   def excluded_from_party?
     false
   end
+
+private
+
+  def supporter_since_not_in_future
+    return if supporter_since.nil?
+
+    errors.add :supporter_since unless supporter_since <= Time.zone.today
+  end
 end
diff --git a/db/migrate/20181215053720_add_supporter_since_to_people.rb b/db/migrate/20181215053720_add_supporter_since_to_people.rb
new file mode 100644
index 0000000..d96d03d
--- /dev/null
+++ b/db/migrate/20181215053720_add_supporter_since_to_people.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddSupporterSinceToPeople < ActiveRecord::Migration[5.2]
+  def change
+    add_column :people, :supporter_since, :date
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 65bd8c8..e7eed19 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2018_12_15_040559) do
+ActiveRecord::Schema.define(version: 2018_12_15_053720) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -127,6 +127,7 @@ ActiveRecord::Schema.define(version: 2018_12_15_040559) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.bigint "regional_office_id"
+    t.date "supporter_since"
     t.index ["regional_office_id"], name: "index_people_on_regional_office_id"
   end
 
diff --git a/spec/models/person_spec.rb b/spec/models/person_spec.rb
index b9a2221..c1fcb29 100644
--- a/spec/models/person_spec.rb
+++ b/spec/models/person_spec.rb
@@ -24,4 +24,20 @@ RSpec.describe Person do
   pending '#party_supporter?'
   pending '#party_member?'
   pending '#excluded_from_party?'
+
+  describe '#supporter_since' do
+    def allow_value(*)
+      super.for :supporter_since
+    end
+
+    it { is_expected.not_to validate_presence_of :supporter_since }
+
+    it { is_expected.to allow_value Time.zone.today }
+    it { is_expected.to allow_value Time.zone.yesterday }
+    it { is_expected.to allow_value rand(10_000).days.ago.to_date }
+
+    it { is_expected.not_to allow_value Time.zone.tomorrow }
+    it { is_expected.not_to allow_value 1.day.from_now.to_date }
+    it { is_expected.not_to allow_value rand(10_000).days.from_now.to_date }
+  end
 end