function to calculate minutes between 2 time fields

function to calculate minutes between 2 time fields

Does anyone know of a function to calculate minutes between 2 time fields? I have 2 input fields, one called LKW which is defined ans a Date/Time field and second is called ER_Arrival which is also defined as Date/Time field. I want to calculate minutes between the 2 fields and present that in a third field called LKW-Door. Can't seem to find a function that will work.