Invalid Last Modified Dates

What is the last modified date as shown in wp-admin for those posts? Also does this happen to all post sitemaps (i.e. from part 1 to part n)?

Hello Khang,
The publication dates of these posts range from 2012 to the present. This issue only occurs from part 5 through 10. Moreover, the last modified date of the Google News sitemap is also -001-11-29 21:00. Kindly refer to https://goo.gl/KeRS9H . Thank you for your assistance.

Can you contact me via: https://betterwp.net/contact/ ?

I might need your help debugging this. Thank you.

Hi Khang,
I send you a message but haven’t been able to reach you. Please let me know if we can troubleshoot this issue for your plugin. Thank you.

I have just replied, thanks.

Many thanks, Khang. Problem solved.
I also left a 5 star review of your plugin.
Thank you.