title: Logs shipped with wrong timestamp and timekey ignored
body: <p>I want to ship my Vault logs to s3. Based on this <a href="https://github.com/fluent/fluent-plugin-s3/issues/187" rel="nofollow noreferrer">issue</a> I did this:</p>
  
  <pre><code>## vault input
  &lt;source&gt;
    @type tail
    path /var/log/vault_audit.log
    pos_file /var/log/td-agent/vault.audit_log.pos
    &lt;parse&gt;
      @type json
    &lt;/parse&gt;
    tag s3.vault.audit
  &lt;/source&gt;
  
  ## s3 output
  &lt;match s3.*.*&gt;
    @type s3
  
    s3_bucket vault
    path logs/
  
    &lt;buffer time&gt;
      @type file
      path /var/log/td-agent/s3
      timekey 30m
      timekey_wait 5m
      chunk_limit_size 256m
    &lt;/buffer&gt;
  
    time_slice_format %Y/%m/%d/%H%M
  &lt;/match&gt;
  </code></pre>
  
  <p>What I'd expect is for my logs to be shipped to S3 every 30 minutes, and be formatted in directories as ie: <code>logs/2019/05/01/1030</code></p>
  
  <p>Instead my logs are shipped every 2-3ish minutes on average, and the output time format in S3 is starting from the epoch ie: <code>logs/1970/01/01/0030_0.gz</code></p>
  
  <p>(the time is correctly set on my system)</p>