What if you combine the the HOUR & MINUTE functions to get the TIME_PART.
Example;
1x Date Attribute
2x Timestamp Attributes
User entered values:
Object.Date (date)
Object.Time - (timestamp) format HH:mm
Calculated Result (timestamp):
Object.DateTime - format dd/MM/yyyy HH:mm
Rule:
If Object.Date IS DEFINED AND Object.Time IS DEFINED Then
Object.DateTime=TIMESTAMP(DAY_OF_MONTH(Object.Date), MONTH(Object.Date), YEAR(Object.Date), HOUR(Object.Time), MINUTE(Object.Time))
By the way, does anyone know how to show AM/PM next top the time when 'display time separately' option is checked. Is it possible?
eg. 
Cheers