Where any time or period has been specified under any of the provisions of the Act or rules made thereunder within which any application is to be made or any act or thing is to be done, the Central Board of Revenue may, in any case or class of cases, permit such application to be made or such act or thing to be done within such time or period as it may consider appropriate:

Provided that the Board may, by notification in the official Gazette, and subject to such limitations or conditions as may be specified therein, empower any Collector to exercise the powers under this section in any case or class of cases.