Does anybody know the best way to set up a trigger for email opens that happen during specific hours? I want to create a trigger campaign for those who open and read our emails at nights. How it's done now:
I was wondering if there's a more elegant way to solve this issue, without the need to use a custom field.
Depends on how you're responding to the activity. Does that requested campaign do something immediately, like send an alert?
You're doing about the best you can with the built-in options.
Certainly a webhook is a more proper fit for such data processing (since it can process date-like strings as dates) but it's not worth the overhead just to save on a single Datetime field.
The most efficient way by far would be to have a Resource Lead call a webhook twice a day, the webhook in turn looping back to activate/deactivate the campaign via the REST API. But again, not worth the extra build unless you're super-comfortable with this kind of work.