What SSR is
SSR is the short sale restriction of SEC Rule 201, part of Regulation SHO. It is a circuit breaker: once a stock falls 10% or more below the prior day's closing price, short sellers can no longer hit the bid.
When it triggers
The trigger price is 90% of the prior day's official close on the listing exchange. A stock that closed at 5.00 triggers at 4.50 or lower. The listing exchange makes the call, and only trades during regular hours, 09:30 to 16:00 ET, count. A pre-market or after-hours print below the trigger does not set it off.
How long it lasts
Once triggered, the restriction holds for the rest of that day and the whole next trading day. Triggered on a Friday, it runs through Monday; a holiday is skipped. It applies whenever quotes are published, so it covers pre-market and after-hours on those days too. If the stock falls 10% below its close again on that next day, it triggers again and the restriction runs through the trading day after.
The uptick rule under SSR
While SSR is on, a short sale may only execute at a price above the current national best bid. You can still short, but you have to offer the stock above the bid and wait for a buyer to lift you; selling short into the bid is blocked. Selling shares you own is not a short sale and is not restricted.
SSR in Hindsight Markets
In the replay, SSR comes from the exchange's own flag for that day, at the moment it was set. Under SSR a short only fills above the bid, and a market or marketable short is refused: rest a short limit above the bid instead. Closing a long position is never restricted.
What it does not do
The tool does the arithmetic from the prices you enter. It does not look up whether a given ticker was on SSR on a past date.