Right format for time of day when corresponding with Germans

I'm an American living in Germany and I'm learning German here, and while Germans never use "am/pm," I think most of them are aware of it and would understand what "10 am" means. Alternately, I don't think there's any doubt that "10.00" (with a decimal as the separator) means 10 am. I think most Germans assume upon seeing time indicated as four digits that it's using a 24 hour clock. Over here I see things like "15.30 Uhr," etc. I'd love to hear what an actual German has to say about this though. I'll ask around in the morning.

As a German native speaker, I would suggest using the full form ("10:00 am") when possible, though the short form should suffice. When no "am" is present, Germans may assume the time is given in 24h form. Germans who are aware of the am/pm distinction in English may be confused when it is missing.

As for the colon vs decimal, I've encountered both forms in German and though I personally prefer the colon, some style guides may disagree (I would have to check the Duden).

So to answer your question: avoid "10 o'clock" as it may be considered ambiguous. As a rule of thumb: if it's before noon, use the "am" form; if it's after noon, use the 24h form or use the "pm" form with the 24h form in parentheses like so:

arrrive at 10 pm (22:00 h)

Alternatively, qualify the time (e.g. "at 10 in the morning") like emragins suggested.

In any case, the American "military time" form ("2200 hrs") will sound odd to most Germans. If there is no separator between the hour and minute in German, it's usually due to laziness, not style.

I agree with Jay's answer from my time in Germany (as an American.) "15.30" was quite common. That said.. since you are writing in English, putting a decimal instead of a colon would then make it improper English.

Were I writing, I would likely out any ambiguity and say "at 10 in the morning."