The fields "PreviousClose" and "PreviousCloseDate" represent the last trade price from the day before. Around 4am each trading day (i.e., a few hours before trading opens), xBATSRealTime and xNASDAQLastSale retrieve the close price for the prior trading day and store it as the previous day close. These values will not be visible in data results until the first trade of the day. Once the first trade is executed for a security, the PreviousClose and PreviousCloseDate fields are filled in and returned with the quote.
Note that PreviousClose represents the official market close (i.e., not just the previous close trade from a given exchange such as BATS or NASDAQ). This is intentional, so that Change/PercentChange reflect the change between the most recent price and the close on the consolidated feed.
If you have any further questions on the PreviousClose data, send your inquiries to firstname.lastname@example.org