Fix Timezone Mismatch in Sit Spec

`Date.today` uses your current time zone, while `Date.yesterday` defaults to UTC.  In various timezones `Date.today` can also be `Date.yesterday` during different times of the day.

- http://pivotallabs.com/date-today-date-yesterday/
- http://stackoverflow.com/questions/8736695/why-does-date-yesterday-counts-as-date-today-also
- http://stackoverflow.com/questions/20637410/date-today-and-date-yesterday-are-the-same-but-not-in-console