Invalid date handling

[cfal]

There doesn't seem to be a way to override the default Date object, while setting the timezone without breaking handling of invalid dates.

The default Date object returns "Invalid Date" for new Date(NaN). Babel's ES6 transpiler does the test (new Date(NaN) + '') == 'Invalid Date', which fails with node-time's extended Date.

> new time.Date(NaN) + ''
TypeError: Cannot read property 'substring' of undefined
    at Date.toDateString (/Users/user/node_modules/time/index.js:471:39)
    at Date.toString (/Users/user/node_modules/time/index.js:484:17)
    at repl:1:20
    at REPLServer.defaultEval (repl.js:252:27)
    at bound (domain.js:287:14)
    at REPLServer.runBound [as eval] (domain.js:300:12)
    at REPLServer.<anonymous> (repl.js:417:12)

(new time.Date(NaN)).toString() gives the same error.

This seems to work when extending the default Date object, but then it breaks setting the timezone using time.tzset:

> var time = require('time')
> new Date()
Thu Mar 31 2016 21:54:15 GMT-0700 (PDT)
> time.tzset('America/New_York')
{ tzname: [ 'EST', 'EDT' ], timezone: 18000, daylight: 1 }
> time(Date)
> new Date()
Thu Mar 31 2016 21:54:36 GMT-0700 (EDT)
> new Date(NaN)
Invalid Date