How do create a validation rule / function to check datetime field is within Business Hours?

How do create a validation rule / function to check datetime field is within Business Hours?

I've got a field called Scheduled_Time.

People keep accidently inputting 2am when they mean 2pm.

Realistically this field is never going to be before 7am or after 9pm.  I'd like to add validation rule to the field (which is a datetime field).

Using the user interface - I can't validate just the time - only the date and time.

So I tried to write a custom function, however I don't seem to be able to access the Scheduled_Date field at all.

Please can you advise if this is possible and, if so, what is the code?

Thanks!