Jekyll on Github suddenly do not work because of 'nil' date - github

Question

Well, I swear I changed no setting at all.
I setup my Jekyll blog, and it did work before.
Every time I want to write something new, I just copy and old post file, rename it to the current date and modify the content within it.
But just one day, after I push my new content to Github, I received an email, noticing me that
The page build failed for the master branch with the following error:
The value 'nil' was passed to a date-related filter that expects valid dates in /_layouts/post.html or one of its layouts. For more information, see https://help.github.com/articles/page-build-failed-date-is-not-a-valid-datetime/.
For information on troubleshooting Jekyll see:
https://help.github.com/articles/troubleshooting-jekyll-builds
First, I did not modify /_layouts/post.html. Second, I do not use date info in YAML, because Jekyll already uses the strong name requirement like YY-MM-DD-title.md.
I am just some guy who wants to write something fun but knows little about IT tech. I searched the Jekyll doc, and read the post on StackOverflow like below:
Invalid date while building on Github Pages
But as my post.html file content is different from that post, I do not know what I can do.
I built it on my Mac with bundle exec Jekyll build, it just shows the same information as the email
$ bundle exec jekyll build
Configuration file: > /Users/zhanglidong/Documents/ASyncFiles/jetorz.github.io/_config.yml
Source: /Users/zhanglidong/Documents/ASyncFiles/jetorz.github.io
Destination: > /Users/zhanglidong/Documents/ASyncFiles/jetorz.github.io/_site
Incremental build: disabled. Enable with --incremental
Generating...
Liquid Exception: Invalid Date: 'nil' is not a valid datetime. in /_layouts/post.html
ERROR: YOUR SITE COULD NOT BE BUILT:
------------------------------------
Invalid Date: 'nil' is not a valid datetime.
My GitHub repo: https://github.com/jetorz/jetorz.github.io
My jekyll blog: https://jetorz.github.io/
How can I fix it? Any one who wants to help the pity me? :(

Answer 1

Like answer above suggested,
I rechecked the link I provided:
Invalid date while building on Github Pages
There is and if block outside the <time> part.
I do not know much about Liquid,
But I think it worth trying.
So I add this if to my post.html like below
{% if page.date %}
<time datetime="{{ page.date | date_to_xmlschema }}" itemprop="datePublished">
{% assign date_format = site.minima.date_format | default: "%b %-d, %Y" %}
{{ page.date | date: date_format }}
</time>
{% endif %}
I do not know exactly what it is about,
but It does work :)
Now again I can post my blog.