Question

Does anyone know why in Firefox if you execute the code below it will validate it as a date if the string passed in is four numbers and only four numbers? In every other browser I tested with (IE, Chrome) it will always return as not a date.

Being that the spec, as pointed out by Marcel Korpel below, states that it should fall back to use the Firefox’s implementation-specific fall back I am really wondering why Firefox’s fall back displays this anomaly.

function isDate(sDate) {  
    var temp = new Date(sDate);  
    if (temp.toString() == "NaN" || temp.toString() == "Invalid Date") {  
        alert("Not a Date");  
    } else {  
        alert("Is a Date!");  
    }
}

Answer 1

If you pass a string to the Date constructor, the string should be in a format recognized by the parse method (IETF-compliant RFC 1123 timestamps) (source: MDC). Everything else results in implementation specific behaviour and will vary across browsers.

I suggest you don’t use strings at all and either use three numbers representing year, month and day (mind that month numbers begin at 0 (= January)), or use one number, the number of milliseconds since 1 January 1970 00:00:00 UTC.

UPDATE: seeing your example,

var a = new Date('0123');
console.log(a);

outputs

Fri Jan 01 0123 01:00:00 GMT+0100 (CET)

so Firefox apparently recognizes '0123' as a year number.

UPDATE 2: I think MDC’s description of Date.parse contains the answer to your question:

Starting in JavaScript 1.8.5, a subset of ISO 8601 formatted date strings can also be parsed.

The ISO 8601 page specifies (section ‘Formats’):

Year:
YYYY (eg 1997)
Year and month:
YYYY-MM (eg 1997-07)
Complete date:
YYYY-MM-DD (eg 1997-07-16)

So when relying on ISO 8601, a string only containing four numbers will be recognized as a year number.