It does look like an issue with coercing an AppleScript date object to an ISO date string - you can just coerce the date to text in your Run AppleScript action. The way you set the time gives an error on my machine (note that as of AppleScript 2.1, a date string must exactly match one of the system date formats), it should be something like:
set FutureDate to (current date) + (6 * days)
set time of FutureDate to (11 * hours)
return FutureDate as text
A-ha, a very intriguing response thank you, I like sticking with modifying the same variable, sadly though running under AppleScript (2.2.1) Editor (2.4.3) and Automator (2.2.4) generates the following error using the revised script, which is puzzling considering the attached observation,
error "" number -1721
However by appending the following to the last line in my earlier script,
fixed the initial script and provided the desired output.