title: How to add portion of timestamp to Fluentd output file format
body: <p>
  How to add certain portion of time stamp to output file generated by fluentd? 
  </p>
  
  <p>In the config file:
  s3_object_key_format generic-logs/%{time_slice}/out-%{H}-%{M}-%{S}-%{index}.log</p>
  
  <p>time_slice_format %Y/%m/%d</p>
  
  <p>
  What i expect for example: out-21-15-45-02.log</p>
  
  <p>
  What i get: out-%H-%M-%S-02.log</p>
  
  <p>Note: I am completely new to fluentd.</p>