Let’s say I have a meeting at 06:00 AM that is scheduled to last 8 hours and 15 mins . Are there any guidelines, tips or tricks to display that information in a non-ambiguous way?

If I simply display in the agenda:

06:00 - Meeting (08:15)

It can easily be misunderstood as 8:15 being either the end if meeting or, even worse, ( since we are using multiple timezones in my app) a different time zone time.

The example is obviously made up, in the real app I have a lot of starting times and a lot of durations to express in a relatively small amount of space on the same page. Simply converting to text (8 hours and 15 minutes) is not an option as it will not fit. I have space for maybe max 10 chars per item.

So how so you display a timespan in a way that is different from a datetime?

Later edit: and just to make the question harder. How about expressing time-to-go, as in the meeting is in 2 hours and 53 minutes. I need an accurate display, and most timers/chrono applications that use countdowns (eg. phone alarm apps) infuriatingly use the same 00:00 display for duration

Format distinction needed

08:15 is typically identified as clock format, not duration.

You could just write (8h) or (8¼h) for duration (see list of fraction unicode chars).

Here is how Google do it:

Duration or endtime?

Another question you should ask is:

Which is more important for users - duration or end time?

If a meeting starts at 06:00 and lasts for 7.5 hours - it takes a bit of mental arithmetic to figure out when it ends (and it is fair to assume users might want to know that as well).

If you only display either duration or end time, there's going to be arithmetic done anyhow.

Perhaps you should just use (start-end):

06:00-14:30

instead?

I see there a debate whether the finish time or duration should be displayed and in fact it's not what the OP is asking for. They decided to go for the duration.

So first of all I suggest making it disambiguous by explicitly stating that the bracketed value is time span:

06:00 - Meeting (for 8:15)

It still looks weird, I'll come back to it later on.

If we were to state the finish time of the meeting, I'd suggest:

06:00 - Meeting (until 14:15)

We already agreed that 06:00 (hours-colon-minutes) notation is regarded as time of day and does not need any explanation unless a clear distinction between 12 and 24 hours format is expected. The former is indicated by a.m./p.m. suffix, the latter can be indicated by adding a leading zero when applicable (thus 06:00 is 6:00 a.m. while 18:00 is 6:00 p.m.)

Now the time span - I propose using one of the following notations (in the order of preference):

8h15'

8h 15m

8¼ h

8.25 h

The first two are widely recognisable since the angular units (' for minutes and " for seconds) are often used on sport watches. The choice will depend on how much space is available.

The third one limits the granularity to a quarter of an hour since common fractions of 1/12 (5 minutes) or 1/6 (10 minutes) are not that intuitive. The former doesn't come up as a single Unicode character and the latter would be ⅙, ⅓, ½, ⅔ and ⅚ for 10, 20, 30, 40 and 50 minutes respectively.

The last notation is computational-friendly but becomes ambiguous when depicting a quarter of an hour (8.25h may by some be seen as 8 hours and 25 minutes) and totally unintuitive and illegible when 5- or 10- minutes intervals come up (0.08(3)h and 0.1(6)h respectively, or 0.08333333h and 0.16666667h).

I therefore recommend you chose between the first two. In such case your notation for the agenda appointment would be:

06:00 - Meeting (for 8h15')

or

06:00 - Meeting (for 8h 15m)

I agree with a previous comment that formatting would make the distinction clear.

08:00 - reads as 8 o'clock (time)

8hrs - reads as 8 hours (duration)

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I would, by all means, NOT follow ISO 8601's guidelines because "P3Y6M4DT12H30M5S" is one of the most confusing strings I've ever seen! As UX designers, we HAVE to come up with a better way to present duration! ;)

6:00 - 14:15 (8hs) - Meeting. (Starts in 2 hours).

The user can easily infer the duration of the meeting from the give time, but you can also display it after the hours. Try not to put descriptions or labels, they will only distract.