Jsonify always returns datetime objects as GMT timezone

[wayneworkman]

Issue

Jsonify always returns datetime objects as GMT timezone. Seems like this has been a problem in the past with these issues:

• jsonify response converts datetime object into string with GMT regardless of objects tzinfo. #2392
• Correctly JSON-encode datetimes aware of non-UTC timezones #2374

Environment

• Python version: 2.7.5
• Flask version: 1.0.2
• Werkzeug version: 0.14.1
• pytz version: 2018.5

Testcode

#!/usr/bin/env python2.7

from werkzeug.http import http_date
from datetime import datetime
import pytz

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/')
def root():
    # Localize time
    local_time = pytz.timezone("America/Chicago")
    time = datetime.utcnow().replace(microsecond=0).replace(tzinfo=pytz.utc)
    time = time.astimezone(local_time)

    print("iso format:", time.isoformat())
    print("object:", time.tzinfo)
    print("http date:", http_date(time))

    result = [time]

    print("Test object: %s" % result)

    return jsonify(result)

app.run(host='127.0.0.1', port=8080, debug=False)

Console output:

Test object: [datetime.datetime(2018, 7, 10, 11, 45, 20, tzinfo=<DstTzInfo 'America/Chicago' CDT-1 day, 19:00:00 DST>)]

Jsonify output:

["Tue, 10 Jul 2018 16:45:20 GMT"]

[davidism]

That's the correct behavior. The default JSONEncoder accepts times with any timezone as input, but always outputs UTC (GMT) in RFC 1123 format.

In the vast majority of cases, you should be working with UTC at boundaries and converting to local time only when displaying to users.

If you want to change the format that dates are serialized, including supporting other timezones, override app.json_encoder with your own flask.json.JSONEncoder subclass.

[jhwinter]

According to this Stack Overflow answer, ISO 8601 is the preferred standard over RFC 1123/822/2822. This link further defines the current best practice.