Then I would evaluate the Date data type in an Eloqua form on the design side and the processing side with Update Contacts with Custom Field Updates whereby there is an update type "Apply Date Stamp" to see if it passes time as part of the value. If it does, I would use that in the form processing step(s).
If not, and trying to keep this simple, then I would evaluate the CMS or site backend to see if they can link to a different Eloqua landing page containing a different form depending on the time of day.
Worst case scenario I would say Eloqua can't do it and have them rethink the text in their email to say "we will respond between normal business hours of 8AM-5PM {Timezone}", "For emergencies please {do this}".
I stand by my previous statement that campaign canvas and program builder are overkill for what should be a conditional form processing step.