Timestamp env variable or argument for telegraf configuration

Question

I am using the inputs.http plugin of Telegraf in order to import data from an API to influxdb. The API requires a time filter in the body of a POST request and responds with data between that time filter. I want to periodically call this API and retrieve data for the past 10 or so seconds. So I need to include the current timestamp in the body of the POST request. Can I pass the current server timestamp to telegraf.conf in the form of an environment variable or a command line argument? What I have attempted so far is using an environment variable in the telegraf.conf file as shown below. It did not work.

[[inputs.http]]
  #URL
  urls = ["url"]

  #http method
  method = "POST"

  ## Optional HTTP headers
  headers = {"cache-control" =  "no-cache","content-type" =  "application/json"}

  ## HTTP entity-body to send with POST/PUT requests.
  #body = "{\"measurement\":\"measurement_name\", \"time_filter\":[1593068400, 1593068800]}"
  body = "{\"measurement\":\"measurement_name\", \"time_filter\":[1593562547, ${date +%s}]}"

  #Data from HTTP in JSON format
  data_format = "json"

I then run the command below

$telegraf -config telegraf.conf

and receive a 400 error. If I replace the body line (includes variable) with the line above it (no variable) everything works fine.