import datetime
import json
import re

rdate = re.compile(r'/Date\((\d+)([+-]\d\d)(\d\d)\)/')

decoder = json.JSONDecoder()
scanstring = decoder.parse_string
def parse_date_or_string(*args):
    s, end = scanstring(*args)
    print(s)
    m = rdate.match(s)
    if not m:
        return s, end
    ts = int(m.group(1)) / 1000
    hroff = int(m.group(2))
    minoff = int(m.group(3))
    off = datetime.timedelta(hours=hroff, minutes=minoff)
    tz = datetime.timezone(off)
    return datetime.datetime.fromtimestamp(ts).replace(tzinfo=tz), end
decoder.parse_string = parse_date_or_string
decoder.scan_once = json.scanner.py_make_scanner(decoder)

print(decoder.decode("""{"timestamp": "/Date(1405961743000+0100)/"}"""))