One of the examples returns [Function] instead of an integer.

I was turning the examples into a test and ran into an error.

```
var ok = require("assert");
var eq = require("assert").equal;
var tz = require("timezone");

var y2k = tz("2000-01-01");
eq( y2k, 946684800000 );
eq( y2k, Date.UTC(2000, 0, 1));

eq( tz("1970-01-01"), 0);
ok( tz("1969-7-21 02:56") < 0);

Thrown:
{ [AssertionError [ERR_ASSERTION]: false == true]
generatedMessage: true,
name: 'AssertionError [ERR_ASSERTION]',
code: 'ERR_ASSERTION',
actual: false,
expected: true,
operator: '==' }
> tz("1969-7-21 02:56")
[Function]
```

I tried it both with a copy I was building for a Debian package, and a copy installed into a new directory with npm install timezone.

Start by reproducing the assertions shown in the issue from the package examples, especially the 1969-7-21 02:56 case, and inspect why the result is displayed as [Function] rather than behaving as a negative number. Done means the example assertions pass and the returned value for that date compares correctly as an integer.

Written by the indexing model from the issue text.