I grew up in South Africa and my whole life I have written time in the format 23h59, 09h12 etc.
Where does the “h” that I use come from as no one I know in the UK has ever seen time written in this format before. The only mention I can find on the net is that they use this format in France. My brother went to a different school than I did and he also writes his time in this format.
It’s a simple way of writing 24-hour time (as opposed to 12-hour (AM-PM) time) if you fear people might confuse hours for minutes and minutes for seconds. It’s (to my eyes) better than any other format I’ve seen, but I’m somewhat uncommon among Americans in preferring 24-hour over 12-hour time. (There are a good deal of Americans like me in this respect, but we’re still an overall minority.)