Read it as a string and parse it into a date with ParseDateTime after the webservice is called.
Update: Create the date yourself with formatDateStr functions like
D = formatDateStr($date, "yyyy-MM-dd") + "T" + formatDateStr($date, "hh:mm:ss")
And use this field in the mapping as a value for the date.
Can't (or I misread your option).
The webservice needs a dateTime and won't except a string. So, I need to create this specific dateTime format structure, otherwise the webservice won't eat it.