Unix Timestamp (Epoch) Converter

A Unix timestamp is a single integer: the number of seconds since the epoch, 1970-01-01T00:00:00Z, defined by POSIX.1-2017 §4.16. Because it carries no time zone, the same number means the same instant everywhere — the converter's only job is to render that instant clearly and to reverse the process exactly.

1. Resolve the unit. On Auto-detect, the digit count decides the unit. The canonical lengths (10 = seconds, 13 = milliseconds, 16 = microseconds, 19 = nanoseconds) each sit inside a band:
   • 1–11 digits → seconds
   • 12–14 digits → milliseconds
   • 15–17 digits → microseconds
   • 18–19 digits → nanoseconds
   You can always override this with the unit selector.
2. Normalise to milliseconds. The value is scaled to milliseconds using exact BigInt arithmetic so a 19-digit nanosecond figure never loses precision to floating point. Microsecond and nanosecond inputs are floor-divided; any sub-millisecond remainder is reported separately rather than rounded into the date.
3. Build the civil date.The millisecond instant is decomposed into a year/month/day and hour/minute/second using UTC fields only, so the host machine's local offset can never leak into a UTC answer. This matches the POSIX relation days = ⌊seconds / 86400⌋ with the remainder giving the time of day, cross-checked against Howard Hinnant's constant-time days-to-civil algorithm.
4. Apply target offsets. UTC is shown with the Z suffix; Sri Lanka adds a fixed +05:30 (Asia/Colombo, no DST since 1996); your local zone uses the offset your browser reports for that exact instant, so historical daylight saving is handled correctly.
5. Format. Each zone is emitted as ISO 8601 / RFC 3339, with an RFC 2822 string for email and HTTP-style dates. The same epoch is echoed in all four units for easy copy-paste.
6. Reverse direction. Date → Epoch parses your calendar input in the chosen zone, converts it to a UTC instant, and reads back the epoch in every unit. The forward and reverse paths are inverses, so a value round-trips to itself.
7. Derived facts. Day of week comes from the UTC weekday; day of year by differencing from 1 January; the ISO-8601 week number uses the Monday-start, first-Thursday rule; and leap years follow the Gregorian rule (y%4==0 && y%100!=0) || y%400==0.

• IEEE Std 1003.1-2017 (POSIX.1) §4.16 — Seconds Since the Epoch
• ISO 8601-1:2019 — Date and time representations
• RFC 3339 — Date and Time on the Internet: Timestamps
• RFC 2822 §3.3 — Date and Time Specification
• IANA Time Zone Database — Asia/Colombo (+05:30)

Conversion logic was last cross-checked against these standards on 2026-06-08. Unix time ignores leap seconds by design (POSIX §4.16), so every day is treated as exactly 86,400 seconds.