I would like to be able to a) specify milliseconds through the editor, b) allow manual input of milliseconds.

Given the below example, a millisecond editor is not rendered for the first scenario, and the milliseconds are reset to 0 on focus-lost for the second scenario.

flatpickr("#date", {
  enableTime: true,
  enableSeconds: true,
  allowInput: true,
  dateFormat: 'Z'
});

See https://jsfiddle.net/ejazs6v1/

v 4.4.6 on Chrome

Thanks!