Hey
I recently had this issue (Using Common Data Service). Here is my code to solve? it.
Text(DateAdd(DateTimeValue(I_StartDate.SelectedDate & " " & I_StartTime),
DateDiff(Now(), DateAdd(Now(),TimeZoneOffset(), Minutes),Hours),
Hours), "[$-en-US]yyyy-mm-ddTHH:mm:ssZ")
From this, you can probably just remove the start time and space i have added.
Let me know how you get on.
Cheers,
Sean