Don't use private `date_` property (see #1026)

It looks like at some point the Closure Library exposed a public `date` property for this same purpose.  The version of the Closure Library that comes with the closure-util package uses this property.  At some point later, I'll demonstrate how we can control which version of the library we depend on.
Test conventions

 * use `be` or `equal` for strict equality tests
 * use `be.a` for `instanceof` checks
 * use dot instead of bracket notation