A passport or travel document shall, unless revoked earlier, continue in force for such period as may be prescribed and different periods may be prescribed for different classes of passports or travel documents or for different categories of passports or travel documents under each such class:
     Provided that a passport or travel document may be issued for a shorter period than the prescribed period--
         (a) if the person by whom it is required so desires; or
         (b) if the passport authority, for reasons to be communicated in writing to the applicant, considers in any case that the passport or travel document should be issued for a shorter period.