Scheduled price changes for online retailers

If a price drop on an item is scheduled for a specific date, when does that price change for online retailers? I would guess that it would be 12:00 am (in the time zone where the online retailer has a physical presence).

In my case, I am looking specifically for Amazon’s policy, but it would also be helpful to know if there is a generally accepted practice as well.

I ask because I am expecting an announced price drop for an item with possibly limited availability. It would be nice to be able to order it as soon as the price changes.