0 - 2 of 2 tags for format change

I have time in format HH:MM and date in format mm/dd/yyyy. I want COL1 with time formated as HH:MM:SS and COL2 a combination of COL1 and date in format MM/DD/YYYY HH:MM:SS.
Also, then I need to subtract COL3 (format mm/dd/yyy hh:mm:ss) from COL2.
How can I do this?

As we know, we have facility to replace NULL values with anystring we want, in Teradata SQL Assistant.
The problem with me is I am fetching the data in a recordset in VBA from MS Excel. In this recordset I am not able to differentiate between NULL and Blank values.