Date Formats — ISO 8601, Unix and Julian Dates
Choose a format for a spreadsheet, decode a production label, or put an exact moment into an API. Compare the conventions before converting your value.
Choose a format with a purpose
Compare formats by their purpose and the information they preserve.
Look up a current format
- Julian Date
Current ordinal Julian code with conversion help and year charts.
- Unix Timestamp
Current epoch time with converter and system background.
- Day of the Year
Current ordinal day and year-progress references.
Understand the standard
- ISO 8601
Machine-readable date and time strings used in APIs, logs, and data exchange.
- Ordinal Date Format
How YYYY-DDD notation works and where it is actually used.
- Seconds vs Milliseconds
Identify timestamp units before they turn into conversion bugs.
Background and edge cases
- Year 2038 Problem
The signed 32-bit overflow issue that still matters in legacy systems.
- What Is Unix Time?
Editorial background on why epoch-based time became a standard.
- ISO 8601 Explained
Why standard date strings matter in software and operations.
A date is not always a timestamp
A birthday such as 1990-07-12 describes a calendar date. It has no inherent time zone. A log entry such as 2026-01-15T14:30:00Z describes one instant worldwide. Converting a birthday to midnight UTC can display the previous date in an American time zone; keep date-only values separate from timestamps.
One moment, several representations
| Representation | Example | Use it for |
|---|---|---|
| Calendar date | 2026-01-15 | Invoices, birthdays, file names |
| UTC timestamp | 2026-01-15T00:00:00Z | Logs and event exchange |
| Ordinal date | 2026-015 | Day-of-year records |
| Unix seconds | 1768435200 | Numeric timestamps |
| Astronomical JD | 2461055.5 | Continuous astronomical day counts |
Check before importing a column
Preserve leading zeros in ordinal codes, identify seconds versus milliseconds, and confirm how missing offsets are interpreted. A spreadsheet may silently convert a code such as 26015 to a number. Import codes as text when their width matters. Use a four-digit year wherever the receiving system allows it.