wordpress contact form 7 hidden date field not working as expected

I am using contact form 7 on wordpress4.5.2. My need is i want to capture post_date (the date on which post has been published) and submission_date(the date on which a candidate submit form.

i am trying to achieve above two needs using following tags which is always returning me 'null'/empty in generated email..some pointers are appreciated.

CF7 form
[hidden today_date today]

[hidden post_date id:post_date]

<p>[submit "Send"]</p>

on mail , below tags used..
1 [_post_date]
2 [post_date] 
[_format_post_date "D, d M y"]
3 [today_date]
[_format_today_date "D, d M y"]
4 [_today_date]

result email..
1 [_post_date]
2 

3

4 [_today_date]

thanks in advance

via Chebli Mohamed