Handling user-significant dates

Presently the user-significant dates in the Tidepool data models are a PwD's birthday and diagnosis date. These dates are calendar dates, and as such they do not need or include any timezone information. Since UTC is an absolute time scale that is also free from the complications of non-universal timezones (such as observance of Daylight Saving Time), using d3-time-format for good examples of this.