Merge Date and Time Variables then send to queue

Hi guys.
I have two variables -
startDate = 12/31/2019 and startTime = 00:00:00
I need to send them together to the orchestrator queue.
Tried to send just startDate as: Convert.ToDateTime(startDate).AddDays(Convert.ToDouble(queueCounter)) → I get 05/14/2020 9:00:00 PM in the orchestrator…
What should I change to add my time variable there?

We can convert it to string right

result=startDate+" "+startTime

So the result will be 12/31/2019 00:00:00


Tried like this:
Convert.ToDateTime(startDate.ToString+" "+startTime.ToString).AddDays(Convert.ToDouble(queueCounter))
But the Orchestrator doesnt recieve this way in POSTPONE…