title: Regex to convert from epoch milliseconds to ISO8601 format?
body: <p>Since I want to do this across a file which has lines beginning with milliseconds representing time series, I was looking for a good regex to use e.g., to make changes across a file or just from a bash shell. 
  Options with <code>sed</code>, <code>awk</code> or with just bash and <code>printf</code> would help.
  Any ideas ?</p>
  
  <p>TIA.</p>