time_read does not include the time a user may have spent reading emails sent from the site. Its value is calculated based on the amount of time the user has spent with topics open on the screen. See How does post tracking work in Discourse for details about how the time is calculated.
You can make API requests to get specific reports. For example, this request works for the User Visits report:
curl -X GET "https://forum.example.com/admin/reports/visits.json?end_date=2019-07-10&start_date=2019-06-10" \
-H "Api-Username: system" \
-H "Api-Key: $api_key" -H \
"Content-Type: multipart/form-data;"
Making requests to the /admin/reports/bulk.json endpoint should work as well. You need to be sure that you add .json to the URL and that start_date and end_date query parameters are added to the URL.