What Is ISO 8601?
ISO 8601 is an international standard for representing dates and times as strings. The format 2024-06-01T00:00:00.000Z is unambiguous, sortable as a plain string, machine-parseable in any language, and explicitly encodes the timezone (Z means UTC). It is the mandated format for REST APIs, OpenAPI specifications, JSON payloads, and most modern databases. When you call new Date().toISOString() in JavaScript or datetime.isoformat() in Python, you get an ISO 8601 string.
ISO 8601 vs Unix Timestamp: When to Use Each
Use Unix timestamps when: you need to do arithmetic (add days, compute durations), you are storing in a database and want fast integer comparisons, you are working with Redis TTLs or AWS S3 expiry, or you are building a high-throughput logging pipeline where string parsing overhead matters.
Use ISO 8601 strings when: you are building a REST API, you are writing JSON payloads that humans might inspect, you are working with Elasticsearch or MongoDB (which store dates as ISO strings), or you need timezone offset information embedded in the value.
The two are interconvertible with no data loss. Many systems use both: store as Unix internally, expose as ISO 8601 in the API response.
Anatomy of an ISO 8601 String
2024-06-01T00:00:00.000Z
│ │ │ │ └ Z = UTC (Zulu time)
│ │ │ └── milliseconds (.000)
│ │ └────────── time (HH:MM:SS)
│ └──────────── T = separator between date and time
└────────────────────── date (YYYY-MM-DD)Converting in Code
// JavaScript
new Date(1717200000 * 1000).toISOString()
// → "2024-06-01T00:00:00.000Z"
// Python
from datetime import datetime, timezone
datetime.fromtimestamp(1717200000, tz=timezone.utc).isoformat()
# → "2024-06-01T00:00:00+00:00"
// Go
time.Unix(1717200000, 0).UTC().Format(time.RFC3339)
// → "2024-06-01T00:00:00Z"Frequently Asked Questions
What is ISO 8601 and why is it used in APIs?
ISO 8601 is the international standard for date-time strings. The format 2024-06-01T00:00:00.000Z is unambiguous (no date format ambiguity like 01/06/2024 vs 06/01/2024), sortable as a plain string, and explicitly carries timezone information (Z = UTC). REST API best practice mandates ISO 8601 for all date-time fields.
What does the Z at the end of an ISO string mean?
Z stands for Zulu time — the UTC timezone (offset +00:00). An ISO string without Z or an offset is technically a 'local' datetime with no timezone, which should be avoided in APIs. You may also see +05:30 or -07:00 as explicit offsets. 2024-06-01T12:00:00Z and 2024-06-01T12:00:00+00:00 are equivalent.
How do I parse an ISO 8601 string in JavaScript?
Pass it directly to the Date constructor: new Date('2024-06-01T00:00:00.000Z'). ISO 8601 strings with a Z suffix are reliably parsed as UTC in all modern browsers and Node.js. Date-only strings like '2024-06-01' are parsed as local midnight in most browsers — always include the T and Z suffix to be safe.
What is the difference between .toISOString() and .toUTCString()?
.toISOString() returns a standard ISO 8601 string like '2024-06-01T00:00:00.000Z' — machine-readable and widely accepted by APIs and databases. .toUTCString() returns a human-readable string like 'Sat, 01 Jun 2024 00:00:00 GMT' in RFC 7231 format, used in HTTP headers. Use .toISOString() for API payloads.
Does ISO 8601 support timezones other than UTC?
Yes. ISO 8601 supports explicit timezone offsets: 2024-06-01T07:30:00+05:30 means 7:30 AM in UTC+5:30 (India Standard Time). JavaScript's Date.toISOString() always outputs UTC (Z). To output with a local offset, use Intl.DateTimeFormat with timeZone option, or a library like date-fns or Luxon.