In this example picture of text, the role of the italics was to de-emphasise the duration.
Generally, italics are used to add emphasis. Am I incorrect in my assumption that this does, actually, de-emphasise it? To my eyes it looks better this way, is it serving a different positive purpose, such as segregating it from the rest of the text?
No. Any alteration in the regular text is going to emphasize it. Even if you reduce the size of the font, it is going to grab the user's attention because it is just different.
I am not convinced that the use of italics here is to deemphasize. When you look at "Yesterday at 2:00:00 PM - Today at 1:00:00 PM" - that doesn't tell you the duration. It tells you when the event ends, but if the duration is what is important giving the start/end time is not going to benefit the user as much. Listing the duration and putting it in italics is going to draw attention to it - since this is (theoretically) more representative of how the user is seeing the task, it is appropriate to draw attention to it.
