Invalid Last Modified Dates

Running into the same problem as this:
https://ww.wp.xz.cn/support/topic/invalid-last-modified-dates

Where my Last Modified dates are coming back like this:
-001-11-30 00:00

For post_part2.xml – 5.xml.

What is the fix? Great plugin, BTW!

Thanks,
Matt

https://ww.wp.xz.cn/plugins/bwp-google-xml-sitemaps/