I'm trying to set up a Drupal Webform that will (among other things), automatically update a custom date field in the connected CiviCRM contact record. The date field in the webform is hidden, and I'm using Drupal Tokens to generate the date/time when the form is submitted; in the webform results I see the correctly generated date and time, but it isn't then passed to the custom date field in the contact record. (I've tried this with both date & time and date being recorded in the CiviCRM custom field, and with different date tokens generated by the webform - none seem to work).
I'm assuming that the webform is generating a date format that can't be interpreted by the custom field, but I can't see a way to match them up.
Other custom fields filled in via the webform are being populated correctly (I haven't tested this extensively).
I'm using Drupal 7.59 and CiviCRM 5.0.2