Changing time settings

And I'm on the newest iOS as well, 11.0.3, and I've not experienced time issues. Your screenshot showed the phone displaying the time correctly, it was the app that was in error. I believe it wouldn't hurt to check with the app developer and see if they are hard coding 12 hour clock to their app if you set the region to US English. Since I don't use those apps, there is no way for me to test it.

I don't use those apps, however the first place I would check is in their settings, then contact their support to see if there is a place to change that, or if it gets its information from the iPhone's clock settings.

I find that answer interesting, since I use 24 hour time format on my phone and do not have any issues. However, I do not use Messenger and Snapchat. Those apps must assume in their settings that US English uses 12-hour format. All my Apple apps, as well as the few others that I use all conform to the phone's time settings.

What troubleshooting have you tried? Have you tried contacting customer support for those particular apps? Since that calendar is not an iPhone app, it is difficult to tell how it determines it. How are those calendar appointments set? Are they from someone else, or do you set them? It could be from the originator that sets them in that time format.