Hi,
I need some help working with dates , adaptive cards and Http requests. Using CoPilot Studio for the first time, so bear with me
My Adaptive card looks like this:

When the user selects a date, it is coming back to me in the format dd/mm/yyyy (even if I try to change the format in the card itself, nothing happens)
In order to send the date to my Http call, I need the date to be in the format yyyy-mm-dd with the time element in the format 'T00:00:00.000Z'
I tried creating a variable setting it's value. But in the Set Variable function, using the datetimeformat gives me the following error
So I tried creating a formula like
Year(Topic.startDate) + "-" + Month(Topic.startDate) + "-" + Day(Topic.startDate) + "T00:00:00.000Z")
But this is falling over with an error that says 'cannot convert '-' into a number' I have tried putting the whole thing into a 'Text' call, individual parts into a Text call. Still get the same error. I'm stumped.
Can somebody please help? Goal is to to take the date entered by the user and send it to the Http request in the format 'YYYY-DD-DDT00:00:00.000Z'
Thanks