This can be done w/ a query to the database. Run this one, and see if your prices will revert with a simple division. If you multilpy, and then divide, it could leave you with prices you didn't expect...

select product_price_id,price,price*1.1 as newprice,(price*1.1)/1.1 as oldprice from product_prices where NOW()>start_time and (NOW()<end_time or isnull(end_time)) and active=1;

That would show you what would be done. You should get someone locally who's familiar with SQL and your database to convert that to an "update" statement to make the changes in the database. You'd want someone to assist locally and ensure nothing bad happens, or who could dig you out of a hole if it does.

There's probably a more correct way to do it by inserting new prices with limited date ranges. This is just a hack that would get you there in a pinch.

Cheers, Paul