Add validator for datetime string

Example usage:

    validates :start_at, date_time_string: true

app/validators/date_time_string_validator.rb

@@ -0,0 +1,63 @@
+# Validates a datetime string with relaxed rules
+#
+# This uses ActiveSupport::TimeZone.parse behind the scenes.
+#
+# https://api.rubyonrails.org/classes/ActiveSupport/TimeZone.html#method-i-parse
+#
+# === Example
+#
+#   class Post
+#     include ActiveModel::Validations
+#
+#     attr_accessor :published_at
+#     validates :published_at, date_time_string: true
+#   end
+#
+#   post = Post.new
+#
+#   post.published_at = nil
+#   post.valid?                 # => true
+#
+#   post.published_at = ""
+#   post.valid?                 # => true
+#
+#   post.published_at = []
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be a string"]
+#
+#   post.published_at = 1
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be a string"]
+#
+#   post.published_at = "2018-09-20 01:02:00 +10:00"
+#   post.valid?                 # => true
+#
+#   post.published_at = "Not Valid"
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be valid"]
+class DateTimeStringValidator < ActiveModel::EachValidator
+  NOT_STRING_ERROR = I18n.t("validators.date_time_string_validator.not_string_error")
+  INVALID_FORMAT_ERROR = I18n.t("validators.date_time_string_validator.invalid_format_error")
+
+  def validate_each(record, attribute, value)
+    return if value.nil? || value == ""
+
+    validate_attribute_is_string(record, attribute, value)
+    validate_attribute_is_datetime_string(record, attribute, value)
+  end
+
+  protected
+
+  def validate_attribute_is_string(record, attribute, value)
+    return if value.is_a?(String)
+
+    record.errors.add(attribute, NOT_STRING_ERROR)
+  end
+
+  def validate_attribute_is_datetime_string(record, attribute, value)
+    return unless value.is_a?(String)
+
+    datetime = Time.zone.parse(value)
+    record.errors.add(attribute, INVALID_FORMAT_ERROR) if datetime.blank?
+  end
+end