DateTime comparison in Rails 6

Hi -

I built this simple app to practice with ActionMailer as I learn Rails (LINK: https://github.com/eleetyson/time-capsule).

It works fine locally but when I deployed to Heroku, it seems that the datetime comparison in the messages controller's create action is not working properly.

Here is the messages controllers (LINK: https://github.com/eleetyson/time-capsule/blob/master/app/controllers/messages_controller.rb) -- I keep hitting that 2nd flash notice ("invalid delivery time...") even when I input a datetime in the future.

Any suggestions for how to debug?

Other relevant links:

• message creation form - https://github.com/eleetyson/time-capsule/blob/master/app/views/messages/new.erb