If you want it to be compact, only mention values that change over the interval.
Don't mention the year unless the interval includes midnight after New Year's Eve.
Don't mention the month unless the interval includes midnight after the last day of a month.
Don't mention the day unless the interval includes midnight.
Users should be able to gather the other units from other parts of the view, so...
- For instance, I can tell from view elements other than your white-on-black text block that the graph is for July and August.
- I actually can't see that it's for 2013, but you can fix that easily by including that in the top bar with the months.
- I can see that the day is the 22nd in this case, but if you are not already doing so, you would want to make sure that there is always a date sitting at the top of your black line (start point marker), as there is now.
All of this means that the majority of cases will involve mentioning just the time (no date) for the start point, and then you have two choices for filling in the rest of the information, of course.
You could give the start point and just the interval duration, as mentioned by Liviu A, for minimum size. Since what I am suggesting would cut down on the amount of text for the start point, though, you could also specify the end point, and the resultant increase in readability should be worth the slight increase in space. Again, in most cases this will just be a time.
examples: ("2013", "July", and "22" gathered from other elements of the view)
- 6:15PM, 15min
a few other choices, if you are serious about compactness:
- ** 6:15–6:30PM (since "PM" is something else that does not change over the interval)
- 6:15–30PM (hour also does not change) (this one might sacrifice readability too much)
- 18:15–18:30 (24-hour time saves space (no AM or PM) and for some users is easier)
- 18:15–30 (most compact, but probably too far)
These should be easy changes from your current design.
(Actually, specifying the end point gives an advantage: Since your largest interval is 1 day, a user will know any interval ends on the day after the start point when the end point is an earlier time. This means that if you choose to show the end point, you will really only need times for all cases, and not just most cases.)