Hidden datetime field?

[xuanswe]

I want to have a hidden field with a default datetime value of now, in format 2025-02-27T18:45:00+01:00 / YYYY-MM-DDTHH:mm:ssZ.

I tried as below, but it doesn't work.

- name: 'lastmod'
  label: 'Last Modification'
  widget: 'hidden'
  default: '{{commit_date}}'

Does anyone know how to do that?
Thank you in advanced!