The appraisee and appraiser can change the date of an appraisal on the appraisal report's Start page:
Select a new date. If you like, Appraisd can email your colleague to let them know about the new date.
Appraisd does not allow you to choose the time of the appraisal. This is left for appraisee and appraiser to do separately.
See also: Editing an appraisal