@Derek_AM, Two alternatives:
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In Timeline, at least on a MacBook Air trackpad, I’m able to scroll and extend the selection vertically (though it takes a little dexterity!), or
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UPDATE: It seems this feature has been removed:
Removed feature
Another approach is in List view. See How to Complete Task Fields in Asana | Product Guide • Asana Product Guide and scroll to the section titled Shift multiple due dates in a list which explains how to move an arbitrary multi-selected set of tasks all a day forward or backward repeatedly.
Larry