Leap year and general date validation for isDate().

[ravestack]

This is a follow up from #418. The one gotcha is that it doesn't accept null as valid which the Date function does. However, this works just fine when I test the function by itself; it may just be an artifact of the testing suite.

Let me know if you'd like to see more tests.

[chriso]

I'm a little bit wary of merging this considering how lenient Date.parse() is and how few tests there currently are. More tests would definitely be appreciated. The ISO8601 tests are a good start: https://github.com/chriso/validator.js/blob/b20a8db53b112eea86dfed9b9d6efabb4cde63de/test/validators.js#L2134

[ravestack]

@chriso Just thought I'd let you know that the ISO 8601 dates in the test suite mismatch in some places with the JavaScript definition of valid date.

For example, these are all valid ISO 8601 dates but invalid JavaScript dates:

'2009123'
'2009-123'
'2009-222'
'2009-W01-1'
'2009-W51-1'
'2009-W511'
'2009-W33'
'2009W511'
'2009-05-19 14'
'2009-W21-2'
'2009-W21-2T01:22'
'2009-139'
'20090621T0545Z'
'2007-04-05T24:00'
'2010-02-18T16:23:48,444'
'2010-02-18T16:23:48,3-06:00'
'2010-02-18T16:23.4'
'2010-02-18T16:23,25'
'2010-02-18T16:23.33+0600'
'2010-02-18T16.23334444'
'2010-02-18T16,2283'
'2009-05-19 143922.500'
'2009-05-19 1439,55'

And these are valid JavaScript dates but invalid ISO 8601 dates:

'200905'
'2009-'
'2009-05-19 14:'
'200912-01'

I've updated my tests with ISO 8601 dates accordingly. I'm sure I need more, but I'd love suggestions as to what type of tests to add.

[chriso]

Awesome, thanks. Going by these Date.parse() docs, some RFC 2282 tests might be helpful.

Not sure if this is a valid timezone offset or not, but isDate() isn't consistent here with Date.parse/new Date:

> validator.isDate('2008-01-01 00:00:00 GMT+0030')
false
> new Date('2008-01-01 00:00:00 GMT+0030')
Tue Jan 01 2008 08:30:00 GMT+0900 (KST)
> new Date(Date.parse('2008-01-01 00:00:00 GMT+0030'))
Tue Jan 01 2008 08:30:00 GMT+0900 (KST)

[chriso]

This isn't necessary. The first argument is always coerced to a string (1, 2, 3) based on these rules: https://github.com/chriso/validator.js#strings-only

[ravestack]

I borrowed some RFC 2822 tests from around the web - I figured it'd be slightly better than trying myself - and I removed tests and checks for non-string types.

I also made sure to only check against actual double digit numbers, so that example you posted should pass.

[chriso]

LGTM, thanks!