Introduce 'optional' validator to epp controllers

app/validators/date_time_iso8601_validator.rb

@@ -0,0 +1,30 @@
+class DateTimeIso8601Validator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    return if self.class.validate(value)
+    record.errors[attribute] << (options[:message] || I18n.t('unknown_expiry_absolute_pattern'))
+  end
+
+  class << self
+    def validate(value)
+      return true if value.empty?
+
+      begin
+        DateTime.parse(value)
+      rescue => _e
+        return false
+      end
+
+      true
+    end
+
+    def validate_epp(obj, value)
+      return if validate(value)
+
+      {
+        code: '2005',
+        msg: I18n.t(:unknown_expiry_absolute_pattern),
+        value: { obj: obj, val: value }
+      }
+    end
+  end
+end