One way to do it is to use a combination of parseDateTime and formatDateTime
e.g. you have two DateTime variables $Date and $Time and you want to take the date component from $Date and the time component from $Time $DateTime = parseDateTime(formatDateTime($Date,'MM-dd-yyy')+' '+formatDateTime($Time,'hh:mm:ss'),'MM-dd-yyy hh:mm:ss')
Not sure if you need to use the UTC versions of parse and format or not, I guess it would depend on whether your dateTime attributes are localized.
I hope this helps,
Disclaimer: I haven't tested this. :)