I am a satisfied owner of a Nokia 925, running Windows Phone 8.1. But one of the things I really disliked, was that I couldn’t get the calendar app to follow the time settings of the regional format I used. My setup at the time was:

Phone Language: English (United States)
Regional Format: Swedish
Keyboard: Swedish

Despite the regional format being Swedish, with the 24h time format was not used in the calendar app. It was annoying to say the least, especially since I never got the hang of the Confusion at noon and midnight issue.

I FINALLY found a “solution” to the problem some time ago, which was to change the language to English (United Kingdom) instead of the US one. I shouldn’t have to change the language in order to get just that one app to use the correct format, but at least I won’t get any reminder alarms going off in the middle of the night anymore.