JSON is a great data format and it's taken the Internet by storm for a number of good reasons. But because of a strange oversight in the EcmaScript specs, there is no standard way of describing dates in JSON. There's been a lot of discussion on this topic and it still remains a problem today.
In the Microsoft Ajax Library, we've tried a number of different approaches before we converged to the solution we're using today.
The first thing we tried was to inject Date constructors in the JSON string. This is a (very) bad idea for a number of reasons. First, it simply does not conform to the JSON specs. Second, any JSON parser that validates its input before parsing it will cough on such a thing. Finally, it establishes a precedent: why would it be allowed for dates and not for arbitrary types? This would just defeat the purpose of JSON.
The second approach, which is what most people settled on, is to adopt some string form of date and just interpret any string that conforms to this format as a date. We've been using "@1163531522089@" where the number is the number of milliseconds since January 1st 1970 UTC for a while (not super-readable) and some people are just using the ISO 8601 format. These are both almost acceptable compromises (if everyone agrees on them) but have a fundamental flaw which is that there can be false positives: what if you want to serialize the "1997-07-16T19:20:30.45+01:00" string, as a real string, not as a date?
To be perfectly honest, JSON Schema does solve the problem by making it possible to "subtype" a string as a date literal, but this is still work in progress and it will take time before any significant adoption is reached.
Our current approach is using a small loophole in the JSON specs. In a JSON string literal, you may (or may not) escape some characters. Among those characters, weirdly enough, there is the slash character ('/'). This is weird because there actually is no reason that I can think of why you'd want to do that. We've used it to our benefit to disambiguate a string from a date literal.
The new format is "\/Date(1198908717056)\/" where the number is again the number of milliseconds since January 1st 1970 UTC. I would gladly agree that this is still not super readable, which could be solved by using ISO 8601 instead.
The point is that this disambiguates a date literal from a string that looks like the same date literal, while remaining pure JSON that will be parsed by any standard JSON parser. Of course, a parser that doesn't know about this convention will just see a string, but parsers that do will be able to parse those as dates without a risk for false positives (except if the originating serializer escaped slashes, but I don't know of one that does).
Here's how you'd encode a string that contains the same literal I described above: "/Date(1198908717056)/".
Notice how a simple eval-based parser will just return the exact same string in both cases, but if you run a simple search for "\\/Date\((-?\d+)\)\\/" and replace with "new Date($1)" before the eval (but after validation), you'll get the dates right in the final object graph. The string I described above will just go through that filter undetected and will remain a string.
We're pretty much satisfied with this solution to the date problem, but of course for the moment very few serializers and parsers support that convention. It would be great if this could become the consensus across the industry.
I'd love to read any comments you may have on that subject.
UPDATE: the regular expression can now find dates before 1970. Thanks to Rick and Marc for the tip.