How can I create the HH:MM between a time field and a date/time field?

ToTimeOfDay (Date/Time t)

Description: Date/Time fields display the date and time. This function returns the TimeOfDay on which the Date/Time t falls in the local time zone.

Example: ToTimeOfDay([Last Modified])

Hope that helps, but I'm not sure I understand your question all the way.

Oh, I think I understand what you are trying to do...

If you want to know the difference in hours and minutes between 2 separate fields, a date/time and just a time (time of day) field, you might do something like this:

Create a formula duration field and make this the formula (you can reverse the order):

ToTimeOfDay([Date/Time Field])-[Time of Day Field]

Then, set your duration field properties to HH:MM. 

That should do it my friend!