serialize json date / microsoft extension

serialize json date / microsoft extension

  • Written by Walter Doekes

  • Published on: 2012-06-21

Bertrand Le Roy describes how Microsoft added a Date object extension to JSON in a compatible fashion to implement serialization and serialization of timezone agnostic datetimes.

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 actually kind of like the hack. So, here is an implementation for Python: (view)


>>> import datetime, json
>>> encoded = json.dumps(
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/json/", line 344, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError:, 6, 21) is not JSON serializable


>>> import datetime, json, jsonext
>>> encoded = json.dumps(, cls=jsonext.ExtendedJSONEncoder)
>>> decoded = json.loads(encoded, cls=jsonext.ExtendedJSONDecoder)
>>> print repr(decoded)
datetime.datetime(2012, 6, 21, 0, 0)

Update 2012-07-09

The v1 version of jsonext did not work with the newer json libs for python 2.7. I’ve updated it to v2 to support both libs. Unfortunately the new style is a giant hack. Apologies in advance.

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