Retrospective amendment power confirmed for central government to amend GST notification, amendments deemed effective from specified dates. The statute declares that a specified government notification under the Central Goods and Services Tax Act shall stand amended and be deemed amended retrospectively in the manner and from the dates set out in the Sixth Schedule, and further deems that the Central Government had the authority to make such retrospective amendments as if it had retrospective amendment power under the relevant GST provisions at all material times.