SQL: Reuse function result in query without using sub-query

In a MySQL DB table that stores sale orders, I have a LastReviewed column that holds the last date and time when the sale order was modified (type timestamp, default value CURRENT_TIMESTAMP). I'd like to plot the number of sales that were modified each day, for the last 90 days, for a particular user.

I'm trying to craft a SELECT that returns the number of days since LastReviewed date, and how many records fall within that range. Below is my query, which works just fine:

SELECT DATEDIFF(CURDATE(), LastReviewed) AS days, COUNT(*) AS number FROM sales
WHERE UserID=123 AND DATEDIFF(CURDATE(),LastReviewed)<=90
GROUP BY days
ORDER BY days ASC

Notice that I am computing the DATEDIFF() as well as CURDATE() multiple times for each record. This seems really ineffective, so I'd like to know how I can reuse the results of the previous computation. The first thing I tried was:

SELECT DATEDIFF(CURDATE(), LastReviewed) AS days, COUNT(*) AS number FROM sales
WHERE UserID=123 AND days<=90
GROUP BY days
ORDER BY days ASC

Error: Unknown column 'days' in 'where clause'. So I started to look around the net. Based on another discussion (Can I reuse a calculated field in a SELECT query?), I next tried the following:

SELECT DATEDIFF(CURDATE(), LastReviewed) AS days, COUNT(*) AS number FROM sales
WHERE UserID=123 AND (SELECT days)<=90
GROUP BY days
ORDER BY days ASC

Error: Unknown column 'days' in 'field list'. I'm also tried the following:

SELECT @days := DATEDIFF(CURDATE(), LastReviewed) AS days, 
       COUNT(*) AS number FROM sales
WHERE UserID=123 AND @days <=90
GROUP BY days
ORDER BY days ASC

The query returns zero result, so @days<=90 seems to return false even though if I put it in the SELECT clause and remove the WHERE clause, I can see some results with @days values below 90.

I've gotten things to work by using a sub-query:

SELECT * FROM (
  SELECT DATEDIFF(CURDATE(),LastReviewed) AS sales , 
         COUNT(*) AS number FROM sales
  WHERE UserID=26
  GROUP BY days
) AS t
WHERE days<=90
ORDER BY days ASC

However I odn't know whether it's the most efficient way. Not to mention that even this solution computes CURDATE() once per record even though its value will be the same from the start to the end of the query. Isn't that wasteful? Am I overthinking this? Help would be welcome.

Note: Mods, should this be on CodeReview? I posted here because the code I'm trying to use doesn't actually work