A chrome extension that colors weekends gray in Google calendar

Available on the Chrome web store here: https://chrome.google.com/webstore/detail/google-calendar-gray-week/anbpifeeedchofljolkgmleojmelihom

This is still very basic, but I've added all the features I'd planned, including a basic color picker. I may add more features if they're requested.

• Not knowing much about JS or Chrome plugins, I borrowed the basic structure for this from https://github.com/imightbeamy/gcal-multical-event-merge
• The only elements that need to be modified are the ones with a data-datekey property, which basically correspond to a single day (there are a few headers that this colors as well, since I think it looks a little nicer)
• The most interesting part of this whole project has been figuring out how the data-datekey property works. It represents a date as mixed-radix number of the form Y*512 + M*32 + D
  • it uses 1/1/1970 as epoch, so given a data-datekey = N = Y*512 + M*32 + D", the actual date corresponding to N is M/D/1970+Y
  • The lowest value of data-datekey, not surprisingly, is for 1/1/1970, though if you navigate there, you'll see that it has data-datekey="33", rather than "0", corresponding to M = 1, D = 1, Y = 0 => 512*0 + 32*1 + 1 = 33
  • Dates before 1/1/1970 are represented using a negative data-datekey. The largest such value 12/31/1969 = 512*-1 + 32*12 + 31 = -97
  • Moreover, having every radix be a power of 2 means that data-datekey can be converted to M/D/Y using only bitwise operators (shifts and masks)

Personal note: I also realized as part of this that dividing by 512 using >>9 actually "fixes" division. If we define the integral quotient idiv(X,Y) to be largest integer Z such that Z*Y <= X, then idiv(-1, 512) = -1, as returned by -1>>9 but not by -1/512. If / worked this way as well, then X%Y would never be negative (but we'd still preserve Y*(X/Y) + (X%Y) = X, which requires X%Y to be negative given the current convention for /). This would have avoided a lot of array indexing bugs that I've written in my life