* `startOfDay(TimeInfo): TimeInfo` returns a new `TimeInfo` representing
midnight at the beginning of the given day.
* `startOfWeek(TimeInfo, WeekDay): TimeInfo` returns a new `TimeInfo`
representing midnight at the beginning of the first day of the week. By
default Monday is used as the start of the week (to be consistent with
`times` view of the day order), but the user can pass in any other day to
"start" the week. Because this find the start of the *current* week, the
returned `TimeInfo` will always be a date in the past or present, never a
date in the future.
