Fix: JSON Date Format

How JSON.stringify() Handles Dates

JSON.stringify() automatically calls .toISOString() on any Date object it encounters:

const event = {
  name: "Launch",
  date: new Date("2024-03-15T10:30:00"),
};

JSON.stringify(event);
// => '{"name":"Launch","date":"2024-03-15T10:30:00.000Z"}'
//                                                    ↑ UTC, always Z suffix

Note: .toISOString() always returns UTC. If you create new Date("2024-03-15T10:30:00") in a local timezone, the serialized string will be converted to UTC.

Why JSON.parse() Returns Strings, Not Dates

const json = '{"name":"Launch","date":"2024-03-15T10:30:00.000Z"}';

const parsed = JSON.parse(json);
console.log(parsed.date);           // "2024-03-15T10:30:00.000Z" ← string
console.log(typeof parsed.date);    // "string", NOT "object"

// This will throw because strings don't have .getFullYear()
parsed.date.getFullYear();
// TypeError: parsed.date.getFullYear is not a function

Reviver Function — Auto-Revive Date Strings

Pass a reviver as the second argument to JSON.parse() to convert ISO strings back to Date objects:

const ISO_DATE_RE = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/;

function dateReviver(key: string, val: unknown): unknown {
  if (typeof val === "string" && ISO_DATE_RE.test(val)) {
    return new Date(val);
  }
  return val;
}

const json = '{"name":"Launch","date":"2024-03-15T10:30:00.000Z"}';
const parsed = JSON.parse(json, dateReviver);

console.log(parsed.date instanceof Date); // true
console.log(parsed.date.getFullYear());   // 2024

Python: Serializing datetime Objects

Python's json.dumps() raises TypeError for datetime objects unless you provide a default handler:

import json
from datetime import datetime

event = {"name": "Launch", "date": datetime(2024, 3, 15, 10, 30)}

# Simple approach: convert everything to string
print(json.dumps(event, default=str))
# => {"name": "Launch", "date": "2024-03-15 10:30:00"}

# ISO 8601 format
print(json.dumps(event, default=lambda x: x.isoformat() if hasattr(x, "isoformat") else str(x)))
# => {"name": "Launch", "date": "2024-03-15T10:30:00"}

# Parse back
raw = json.loads('{"date": "2024-03-15T10:30:00"}')
dt = datetime.fromisoformat(raw["date"])
print(dt.year)  # 2024

ISO 8601 Format Reference

Timezone Gotchas

// Beware: date-only strings are parsed as UTC midnight
new Date("2024-03-15").toISOString();
// => "2024-03-15T00:00:00.000Z"  ← UTC

// But date+time without Z is parsed as LOCAL time
new Date("2024-03-15T10:30:00").toISOString();
// => "2024-03-15T05:00:00.000Z"  ← shifted if you are in UTC+5:30

// Always append Z or an offset for unambiguous UTC:
new Date("2024-03-15T10:30:00Z").toISOString();
// => "2024-03-15T10:30:00.000Z"  ← correct UTC

Inspect Your JSON Below

Paste JSON containing date strings to validate the format.