Hi Bim.  I think I was able to figure it out.  Try the steps below and let me know if it works for you.

1. Add a "Calculated Value" control to the form.
2. Set the formula for the "Calculated Value" control to:  formatDate(DateControlName, "hhmm")
   1. DateControlName is the name of your Date/Time control on the form.
   2. The result of the "Calculated Value" control would be "0000" by default.
3. Add a form validation rule to your Date/Time control and configure it like:
   1. Rule Type = Validation
   2. Condition = contains(CalcValueControl, "0000") where CalcValueControl is the name of the control created in Step 2.
   3. Message = Enter your message
4. That should make the Date/Time input control invalid if the date/time is not changed.
5. Also, you'll probably want to create a rule to hide the "Calculated Value" control.

Hi Eric

This is what I already tried and strangely it does not work at all!
The calculated field does not show any figures when running a test ....

However, the calculated value shows the value only when the form has been submitted.
Therefore, I think there is a JavaScript which forces the field to automatically update but unsure how that is done

The BookingDateTime format is set to: Standard.

UPDATE

I have come across the following article in the forum: Validate Time portion of date/time control in a form

whereby Marian Hatala suggested the following formula as a validation

equals(formatDate(StartDate,'hhtt'),'12AM')

In my case, I am using the 24hr date format and had to change the formula as following:

equals(formatDate(BookingDateTime,'HHmm'),'0000')

RESULT

in hope there won't a be a flight at 00:00 lol