use the existing method and update validations

app/models/scheme_deactivation_period.rb

@@ -1,4 +1,6 @@
 class SchemeDeactivationPeriodValidator < ActiveModel::Validator
+  include CollectionTimeHelper
+
   def validate(record)
     scheme = record.scheme
     recent_deactivation = scheme.scheme_deactivation_periods.deactivations_without_reactivation.first
@@ -24,6 +26,8 @@ class SchemeDeactivationPeriodValidator < ActiveModel::Validator
   end
 
   def validate_deactivation(record, scheme)
+    earliest_possible_deactivation = FormHandler.instance.in_crossover_period? ? previous_collection_start_date : current_collection_start_date
+
     if record.deactivation_date.blank?
       if record.deactivation_date_type.blank?
         record.errors.add(:deactivation_date_type, message: I18n.t("validations.scheme.toggle_date.not_selected"))
@@ -32,10 +36,10 @@ class SchemeDeactivationPeriodValidator < ActiveModel::Validator
       end
     elsif scheme.scheme_deactivation_periods.any? { |period| period.reactivation_date.present? && record.deactivation_date.between?(period.deactivation_date, period.reactivation_date - 1.day) }
       record.errors.add(:deactivation_date, message: I18n.t("validations.scheme.deactivation.during_deactivated_period"))
-    else
-      unless record.deactivation_date.between?(scheme.available_from, Time.zone.local(2200, 1, 1))
-        record.errors.add(:deactivation_date, message: I18n.t("validations.scheme.toggle_date.out_of_range", date: scheme.available_from.to_formatted_s(:govuk_date)))
-      end
+    elsif record.deactivation_date.before? earliest_possible_deactivation
+      record.errors.add(:deactivation_date, message: I18n.t("validations.scheme.toggle_date.out_of_range", date: earliest_possible_deactivation.to_formatted_s(:govuk_date)))
+    elsif record.deactivation_date.before? scheme.available_from
+      record.errors.add(:deactivation_date, message: I18n.t("validations.scheme.toggle_date.before_creation", date: scheme.available_from.to_formatted_s(:govuk_date)))
     end
   end
 end

config/locales/en.yml

@@ -451,6 +451,7 @@ en:
       toggle_date:
         not_selected: "Select one of the options"
         invalid: "Enter a valid day, month and year"
+        before_creation: "The scheme cannot be deactivated before %{date}, the start of the collection year when it was created"
         out_of_range: "The date must be on or after the %{date}"
       reactivation:
         before_deactivation: "This scheme was deactivated on %{date}. The reactivation date must be on or after deactivation date"