SQL - Querying Data Within Certain Time Ranges

Question

I'm trying to create a table called "Stats" that employees use a business application where they perform searches and projects on it. I'm trying to answer the following questions with the SQL queries:

1. How many different employees used the business application within a certain time range?

2. How many searches were performed within a certain time range?

3. What was the average search time within this time range?

Here's the table:

CREATE TABLE Stats (
  employee_id INT PRIMARY KEY,
  number_of_employees INT,
  search VARCHAR(50),
  number_of_results INT,
  start_time timestamp with time zone,
  completion_time timestamp with time zone
)

INSERT INTO Stats (employee_id, number_of_employees, search,    number_of_results, start_time, completion_time)
  VALUES (001, 125, “organization”, 2000, 2020-04-18 12:13:21.101+00, 2020-04-18 12:13:26.53+00)
INSERT INTO Stats (employee_id, number_of_employees, search, number_of_results, start_time, completion_time)
  VALUES (005, 127, “organization”, 2000, 2020-04-18 12:14:32.877+00, 2020-04-18 12:16:53.43+00)

I tried to query the distinct number of employees by creating parameters to find the needed info within the time range, but I keep getting errors and I don't know why? Here's what I have:

    SELECT DISTINCT COUNT(employee_id), COUNT("search"),
    AVG(number_of_results), COUNT(*), start_time, completion_time
    FROM Stats
    WHERE employee_id =
    (
    SELECT employee_id, ROW_NUMBER() OVER(PARTITION BY "search")
    FROM Stats
    );
    CREATE PROCEDURE timerange(
    StartDATE = date(start_time),
    EndDATE = date(completion_time),
    StartTIME = timestamp(start_time),
    EndTIME = timestamp(completion_time)
    DATEDifference DATE = EndDATE - StartDATE
    TimeDifference TIME = ENDTIME - StartTIME
    CertainRange = CONCAT(DateDifference || TimeDifference) AS
    CertainTimeRange
    )
    LANGUAGE plpgsql
    AS $$
    BEGIN
    (SELECT CertainRange
    FROM Stats
    WHERE start_time >= StartDATE 
       AND completion_time >= EndDATE 
       AND completion_time >= EndDATE 
       AND completion_time >= EndTIME
    ORDER BY employee_id ASC LIMIT 2);
    END;
    $$;

I get an error when I use the time function. I'm not able to grab the users, "search" results, and the average search time. I'm not quite sure if I need to alter the table to create a new column that does a DATEDIFF between the date parameters in order to hold the certain time range. I'm also not sure how to subtract the difference for timestamps. Please help, I'm really stuck here.