The seems like the date/time wire format used in WCF. From MSDN it states:
DateTime values appear as JSON strings in the form of
“/Date(700000+0500)/”, where the first number (700000 in the example
provided) is the number of milliseconds in the GMT time zone, regular
(non-daylight savings) time since midnight, January 1, 1970. The
number may be negative to represent earlier times. The part that
consists of “+0500” in the example is optional and indicates that the
time is of the Local kind – that is, should be converted to the local
time zone on deserialization. If it is absent, the time is
deserialized as Utc. The actual number (“0500” in this example) and
its sign (+ or -) are ignored.
If so, this issue has been discussed several times on SO.
.net JSON Date format
POSTing a DateTime from Android to a WCF RESTful JSON Service
solved Transform Date to JSON