Does that make my approach any better? I think that addresses every issue. I agree, though, there are better ways to do this, but this isn't a priority project for me so I haven't put as much thought into it as I'd like. Thanks, Neal.
To borrow MVC terminology, your DOM element is then effectively a controller + view in one. Just like we might translate a slider at 50% into 128/256, it's okay to take its properties and then translate those into your model. Presumably you poll the DOM object as it is being dragged, or fire off events when it stops being dragged, which updates the model in near-real-time. Do I understand it?
This is actually a good idea then. Although you want to be careful that your controller doesn't rely on magic constants to do its conversion, it should derive those from some initialization from something representing a "layout". But you seem to have done this.
To get back to your original question... I did find the code hard to read. I don't have the time to unravel it to make it clearer, but personally, I always like to state the problem as clearly as I can up front, in an expression that approaches pseudocode. Like "return getStartOfWeekTime(el) + getOffsetTime(el)". This hides away the hard bits in smaller routines, and the clueless maintenance programmer (i.e. you in one month) will understand what's going on right away and where the bugs might be. But if this is too slow you may have to bite the bullet and live with something hard to read.
this.hSpacing = Array.prototype.slice.call(this.parent.blocks.filter(
function(n){ return n < _this.parent.cols; })
.map(function(n,o){ return o.offsetLeft; })
);
This seems like a very bad design. Is it really better than simply attaching a time property to every div at calendar creation? You are coupling presentation and data, which is in general a terrible idea.
Also, you realize that other countries arrange days of the week differently? If you ever internationalize, you will be in a world of pain. If you ever decide you want column width to vary for some reason, again you are fucked.
You can sometimes justify tricky Javascript since download speed is a factor. However, your JS strikes me as probably more wordy and complex than the straightforward solution.
Tricky techniques that exploit tight coupling are almost always wrong.