Add helper text to datetime-local form fields

app/helpers/maintenance_tasks/tasks_helper.rb

@@ -109,7 +109,7 @@ def parameter_field(form_builder, parameter_name)
       when ActiveModel::Type::Decimal, ActiveModel::Type::Float
         form_builder.number_field(parameter_name, { step: "any" })
       when ActiveModel::Type::DateTime
-        form_builder.datetime_field(parameter_name)
+        form_builder.datetime_field(parameter_name) + datetime_field_help_text
       when ActiveModel::Type::Date
         form_builder.date_field(parameter_name)
       when ActiveModel::Type::Time
@@ -120,5 +120,19 @@ def parameter_field(form_builder, parameter_name)
         form_builder.text_area(parameter_name, class: "textarea")
       end
     end
+
+    # Return helper text for the datetime-local form field.
+    def datetime_field_help_text
+      text =
+        if Time.zone_default.nil? || Time.zone_default.name == "UTC"
+          "Timezone: UTC."
+        else
+          "Timezone: #{Time.now.zone}."
+        end
+      tag.div(
+        tag.p(text),
+        class: "content is-small",
+      )
+    end
   end
 end

test/dummy/app/tasks/maintenance/params_task.rb

@@ -29,7 +29,7 @@ def collection
     def process(post)
       sleep(1) unless self.class.fast_task
 
-      post.update!(content: "New content added on #{Time.now.utc}")
+      post.update!(content: "New content added on #{Time.now.utc}:\ndatetime_attr: #{datetime_attr.inspect}")
     end
 
     private

test/helpers/maintenance_tasks/tasks_helper_test.rb

@@ -90,5 +90,59 @@ class TasksHelperTest < ActionView::TestCase
       assert_match %r{rails/active_storage/blobs/\S+/sample.csv},
         csv_file_download_path(run)
     end
+
+    test "#parameter_field adds information about datetime fields when Time.zone_default is not set" do
+      with_zone_default(nil) do
+        markup = render(inline: <<~TEMPLATE)
+          <%= fields_for(Maintenance::ParamsTask.new) do |form| %>
+            <%= parameter_field(form, 'datetime_attr') %>
+          <% end %>
+        TEMPLATE
+        assert_match "UTC", markup
+      end
+    end
+
+    test "#datetime_field_help_text is correct about the timezone when Time.zone_default is not set/default" do
+      with_zone_default(nil) do
+        value = ActiveModel::Type::DateTime.new.cast("2023-06-01T12:00:00") # a local ISO8601 time (no timezone)
+        assert_predicate(value, :utc?) # uses UTC
+      end
+    end
+
+    test "#parameter_field adds information about datetime fields when Time.zone_default is set" do
+      with_zone_default(Time.find_zone!("EST")) do # ignored
+        markup = render(inline: <<~TEMPLATE)
+          <%= fields_for(Maintenance::ParamsTask.new) do |form| %>
+            <%= parameter_field(form, 'datetime_attr') %>
+          <% end %>
+        TEMPLATE
+        assert_match Time.now.zone.to_s, markup
+      end
+    end
+
+    test "#datetime_field_help_text is correct about the timezone when Time.zone_default is set" do
+      now = Time.now
+      sign = now.utc_offset > 0 ? -1 : 1 # make sure we don't overflow
+      sign = -1
+      # Time.use_zone leaks Time.zone_default in the IsolatedExecutionState, so it needs to be called before changing
+      # Time.zone_default
+      Time.use_zone(now.utc_offset + 2.hour * sign) do # a timezone that is not the system timezone
+        with_zone_default(Time.find_zone!(now.utc_offset + 1.hour * sign)) do # another non-system timezone
+          value = ActiveModel::Type::DateTime.new.cast("2023-06-01T12:00:00") # a local ISO8601 time (no timezone)
+          refute_predicate(value, :utc?)
+          assert_equal(now.utc_offset, value.utc_offset) # uses system time zone
+        end
+      end
+    end
+
+    private
+
+    def with_zone_default(new_zone)
+      zone = Time.zone_default
+      Time.zone_default = new_zone
+      yield
+    ensure
+      Time.zone_default = zone
+    end
   end
 end