Hi Kara,
Just to check, you want the date/time question to match the current date/time each time the survey is open/edited, regardless of the time that was saved in the Favorite answers, have you tired using the now() function in the calculation field?
There is a difference in the way that defaults and calculations are applied to the survey, defaults only get loaded on initial startup of the survey, however calculations are evaluated and applied every time something changes int he survey.
Regards,
Phil.