When you edit a week or day view's Format > Settings, you can change Time Grouping to 'half hour'. After you've done that, however, all items with a from-to date on the calendar still show as if the calendar were grouped by hour.
Attached image will show you what I mean.
To "fix" the display in the screen shot, I had to edit this piece of html:
Old, broken version
<div class="item d_4 o_1 i_0 md_0 first_item">
New, working version
<div class="item d_8 o_2 i_0 md_0 first_item">
As you can see, the display is indeed taking the 'half hour' Time Grouping into account, but the CSS class integers applied to the actual calendar items aren't multiplied to reflect this change. They still show as if the calendar were grouped by hour.
Suggested fix: do some calculations based on Time Grouping before applying classes to style calendar items. Additionally, having at least support for quarter hours would be great in case it's too difficult to support custom Time Grouping like this.