title: Convert julian day into date
body: <p>I have files named day00000.nc, day00001.nc, day00002.nc, ...day00364.nc for several years.
  They represent the 365 or 366  days.
  I want to rename my files like this day20070101.nc, day20070102.nc , ...day20071231.nc
  How can I do that ? 
  Thank you</p>