I'm guessing what is happening here is the theme is using datetime from the filename, which will only get you the month, day, and year (eg. YYYY-MM-DD).
If you want hours and minutes you'll need to add something like this to your post's YAML Front matter.
I've never used jekyll-compose so no idea if it can do that when it creates the post. But without it the theme will just fallback to
00:00 since it has no clue what the hour or minutes are on the date.