title: Trigger alert in prometheus/grafana for timestamp from node-exporter too old
body: <p>We've got node_exporter running, and I am using the textfile collector to let me put in a timestamp of a file modified by a job.</p>
  
  <p>The metrics output: </p>
  
  <pre><code># HELP job_timestamp_file Metric read from /usr/local/etc/node_exporter/job-timestamp.prom
  # TYPE job_timestamp_file untyped
  job_timestamp_file 1.567657381e+09
  </code></pre>
  
  <p>So we've successfully collected that in prometheus.  What my prom guy and I are having problems with, both being pretty new to prometheus and grafana, is how to write the query to trigger an alert if</p>
  
  <p>current-epoch-time - job_timestamp_file > 86400</p>
  
  <p>Which really seems like it ought to be easy...</p>