The appraisal period starts on the date that the last appraisal period ended and ends on the date the performance appraisal is conducted. Normally, for an annual appraisal, this is one year. Or, if this is the employee's first appraisal the period starts on the hire date and ends on the date the appraisal is conducted. If no appraisal was conducted last year, then this appraisal should represent the last 12 months.