Efficiently computing aggregate functions over subqueries with incremental data
The standard deviation can be calculated knowing the number of values, the sum of the values, the sum of the square of the values. Each of these can be updated incrementally as new values arrive and stored in a work table. The work table will look something like

DailyTotals (
  OrderDate,
  NumberOfValues,
  SumOfValues,
  SumOfSquareOfValues);


Since the work table is keyed by date, your desired fourteen day sliding window can be achieved. Since each value is a sum, summing again for each date's value is not a mathematical problem. Yes, there is a calculation at runtime. It is much lighter than the full standard deviation one, however.

When new values arrive the work table can updated synchronously (it's a 1-row update), or asynchronously or in batch depending on the application's appetite for stale data.