Type Mismatch: 'FormatDateTime'

I am trying to extract the time out of a date/time but recieve the type mismatch error. The data is stored in an Oracle database as a Date/Time and I am able to extract it from the DB and display it on the screen OK, but when I try and format it like this:

Dim timeofpurchase
timeofpurchase = FormatDateTime(rscustomer("time_of_purchase"), 4)

I get an error. I have had a look at documentation on the web but all of the examples only show how to format the current date and time, not how to format a variable that contains a date.

My question is, how do I force my variable, rscustomer("time_of_purchase"), to be treated as a time?? or how do I declare the variable timeofpurchase to accept a date/time??