PHP datetime to W3CDTF like 1994-11-05T08:15:30-05:00 syntax for RSS or Atom feeds

I have date time like 2017-07-23 10:05:50 but i need 2017-07-23T10:05:50-05:00

I have try with --

date('Y-m-dTH:i:s', strtotime($old_date)); // Return : 2017-07-23CEST10:26:41

But this is not valid https://validator.w3.org/feed/docs/error/InvalidW3CDTFDate.html