Try this:
concat(left(datefield,10),left(timefield,8))
10 char on date field based on full date yyyy-MM-dd.
8 char on time field based on full time hh:mm:ss.
It depends on the format you want it. normally you can use script above and you can concat another field or string as you want it.
Because actually date and time field tread as string if you read it. But of course you will got error while update or insert it.