I use rails and haml, haml-rails.
How I create hidden_field in haml?
I have field:
.field
= f.label :submit_date
= f.datetime_select :submit_date
I will hide this field. User not able to modify datetime manualy. I will store in db datetime when object was created. I have absolutely no idea how to do it. I know how to do it in erb, but i will use haml.
If you want to create a hidden field with say the customer_id, you'd:
= f.hidden_field :customer_id, value: @customer.id
Note: you mention that you want to store a created_at when the object is created. If you have "timestamps" on your model, or "created_at" and "updated_at" columns, rails will take care of this for you (and is the recommended/standard practice)