See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
What happens is sometimes, USCIS by mistake gives you more time than you are entitled soon for your H-1. Let's say your 6 years is getting over in one year and they give you 2 or 3 years by mistake.
Can you use that time?
And the answer is, that is very risky, don't do that because USCIS could say that you are deliberately misusing an error made by them. They can consider it to be a deport-able offense. It can be a mess. You should assume you have to go through whatever the correct date. You can even call USCIS, make notes of date and time of call etc. Ask them what they want you to do. This is something that your lawyer should be able to resolve. Have them talk with USCIS, protect yourself. Don't assume that because USCIS made an error and gave you more time, you can use it.