Fix timezone fill issue by scaling 'width' with UTC dates, instead of local dates

# What? Why?
Width calculation was weird around DST, primarily because we were using local dates and plugging them into a `d3.utc` time scale. I'm sure there is a better way to fix this issue, but I'm no pro w/ js dates :(

# How was it tested?
Scrolled to March 8/9 boundary, and saw that there is not white space in the fill color.

Note: this fixes the issue w/ safari, mainly because of the findNearestHourForDate helper function I added.

@jebeck