He means the new stock Calendar in WP8.1. I have the same issue.
I believe this is due to the US-centric nature of the Preview for Developers. Same way as Cortana requires the phone to be set to US and automatically displays temperature in Fahrenheit.
It'll probably support all formats when it is officially released, including the 24h clock.
When using US region with non US regional settings dates are display with the 24h format except in the calendar app. It's a bug in the app itself and I don't think there's anything to do about it from the user side.
When using US region with non US regional settings dates are display with the 24h format except in the calendar app. It's a bug in the app itself and I don't think there's anything to do about it from the user side.
UK Settings
Even though the calendar showed times in 24 hour format when entering times I had to remember to check the am/pm setting.
After setting an appointment for 7am instead of 7pm due to there being no 24 hour clock I tried changing the language settings with no sucess till I chose Welsh UK as the language and it now works properly; I just have to learn the welsh for days of the week and months of the year!
What this does show however is that it is possible, and I still have ? signs