Вот решение, которое я придумал, используя событие 'RowInserted':
protected void EPTimeCardDetail_RowInserted(PXCache sender, PXRowInsertedEventArgs e)
{
var eptcd = (EPTimecardDetail)e.Row;
DateTime theDate = (DateTime)eptcd.Date;
DateTime MyDate = new DateTime(theDate.Year, theDate.Month, theDate.Day, 8, 0, 0);
eptcd.Date = MyDate;
}