This basically a forked repo of Logseq Plugin sample with a couple of enhancements:
This should be somehow a core feature as stated in here. In that way, the look and feel can be ported over from the main custom.css.
I actually use the current version to create journal pages for future dates. I find it very helpful. You see that feature as a bug?
I only started using it, but I expect it to add a new page if it doesn’t exist already, as well.
I don’t understand in how far the generated page be invalid?
When I used it (a few versions of Logseq back ago) - it created a journal page which when reindexed, the file goes missing. Even the journal date format wasn’t as per the settings. Not sure if this is related to the Logseq version or desktop app OS-related or I did something stupid while testing.
After that, I stopped using the plugin until recently I had the time to change the way it behaves. If the plugin wants to introduce such a feature, I think it should have a prompt about creating a new journal page.
IMO journal should be something to be used for daily notes and etc. - if we want to place a reminder or something for future dates, Logseq has the TODO workflow and queries - can be placed in the journal template, and every day the query will pull up what’s there under TODO for that particular date.
That’s just my opinion, but of course, we can use the app as to how we see it fit. 
I haven’t had that issue, and I use this feature almost every day, as part of this workflow.
This feature doesn’t work with me – it creates a file with different date format (I use dd-MM-yyyy).
But I am looking forward to whenever this is fixed. This is due to the fact that logseq plugin api does not expose preferred date format yet (AppUserConfigs | @logseq/libs). According to Charlie, this feature will be released in the next API version.
I would hypothesize that this is the root to @mogavenasam’s issue with the invalid journal page.
Hah ah! That explains why everyone thinks that it’s a cool feature and it’s a bug for me. lol