Unix timestamp 0 is January 1, 1970 at 00:00:00 UTC — a choice made in the early 1970s that seemed arbitrary at the time but has become so foundational that billions of systems simultaneously reached the milestone of "1 billion seconds since the Unix epoch" in 2001 and nobody particularly noticed
The previous articles on this site covered Unix timestamps and timezone bugs, ISO 8601 date handling mistakes, and Y2K/Y2K38/Excel's leap year bug. This article addresses why the Unix epoch was chosen, the Y2K38 problem in more detail than the previous article covered, and the practical question of what happens to timestamps in negative territory (dates before 1970).
Why January 1, 1970: the accidental standard
The Unix epoch (Unix time = 0 = January 1, 1970, 00:00:00 UTC) wasn't chosen for any particularly deep reason. Early Unix systems were developed starting around 1969-1970; the epoch needed to be a recent, convenient reference date for the time.
Earlier Unix systems actually used different epochs. The first Unix timestamp epoch was January 1, 1971 (some sources say January 1, 1972); the date shifted across early Unix versions as systems were redesigned. January 1, 1970 emerged as the standard with broader Unix adoption.
Why it matters that the choice was arbitrary: it means every language, database, and system that subsequently adopted "Unix time" inherited this specific reference point — not because January 1, 1970 has any intrinsic significance, but because Unix had it and everything else followed Unix. Today, Unix time is the de facto standard for machine-readable time in virtually every programming language, database, and API.
Negative timestamps: before 1970
Unix timestamps can be negative — representing dates and times before the epoch. −1 is December 31, 1969, 23:59:59 UTC; −86400 is December 31, 1969, 00:00:00 UTC.
Whether negative timestamps work correctly depends on the implementation:
In most modern 64-bit systems: negative timestamps work correctly — they're signed 64-bit integers, and negative values represent pre-1970 dates with correct arithmetic.
In older 32-bit systems or systems that treated timestamps as unsigned: negative timestamps might be interpreted as very large positive numbers (wrapping around), producing dates far in the future rather than dates in 1969.
In many programming languages, databases, and APIs: negative timestamps technically work but may have bugs. PHP historically had issues with negative timestamps. Some databases restrict timestamp columns to the "Unix timestamp era" (January 1, 1970 onward) while offering separate "date" types for historical dates.
Practical implication: if you're storing timestamps for events that might have occurred before 1970 (historical records, dates of birth for elderly users, vintage product manufacture dates), test explicitly with pre-1970 dates to verify your stack handles them correctly.
The Y2K38 problem: detailed mechanics
32-bit signed integers max out at 2,147,483,647 — which, as a Unix timestamp, is January 19, 2038, 03:14:07 UTC. After this second, a 32-bit signed integer representing a Unix timestamp overflows to -2,147,483,648 — which is December 13, 1901, 20:45:52 UTC.
The previous "Y2K/Y2K38/Excel" article introduced this concept; here's the more detailed picture of where it actually affects systems:
Systems already affected: any code that calculates a future timestamp using 32-bit arithmetic will overflow before 2038 if the calculated date falls past the limit. If your code calculates "current time + 30 years" for an expiry date, and you're running this code in 2009, you're already producing overflowed (wrong) dates.
Systems that will be affected in 2038 itself: anything storing current time in a 32-bit integer field — database columns, file metadata timestamps, network protocol fields. Time_t on 32-bit Linux was historically a 32-bit signed integer.
Status of mitigations: major operating systems (Linux, macOS, Windows) have already transitioned 64-bit builds to 64-bit time_t, pushing the Y2K38 boundary to approximately 292 billion years from now. The risk is concentrated in embedded systems (security cameras, industrial controllers, network equipment) that run 32-bit code and may not be updated or replaced before 2038.
Millisecond and nanosecond timestamps: the JavaScript case
JavaScript's Date.now() returns milliseconds since the Unix epoch, not seconds — giving 13-digit timestamps for current dates rather than the 10-digit second timestamps more common in Unix/C contexts.
This creates a common conversion bug: a Unix timestamp in seconds passed to a JavaScript Date constructor as-is (which expects milliseconds) produces a date in 1970, a few seconds past the epoch. Multiplying by 1,000 is required.
The reverse: a JavaScript millisecond timestamp divided by 1,000 (or passed to a Python/PHP/database context expecting seconds) produces the correct date but loses millisecond precision.
Knowing whether a timestamp is in seconds, milliseconds, or microseconds/nanoseconds (the last two are common in high-precision logging systems) requires either documentation or range checking:
- 10-digit timestamp: seconds (current range: ~1.7 billion)
- 13-digit timestamp: milliseconds (current range: ~1.7 trillion)
- 16-digit timestamp: microseconds
- 19-digit timestamp: nanoseconds
How to use the Timestamp Converter on sadiqbd.com
- Convert between Unix timestamps and human-readable dates — the core function. Useful for debugging API responses, log files, and database values that show raw timestamps
- Specify the timezone for the output date — Unix timestamps are always UTC; the human-readable interpretation depends on which timezone you're converting to. "1700000000" is the same moment everywhere; what local time it represents depends on where you are
- For 13-digit timestamps (JavaScript milliseconds): divide by 1,000 before entering, or check if the tool accepts millisecond timestamps directly
Frequently Asked Questions
Is there any way to know whether a number is a Unix timestamp just by looking at it? By range, approximately. A 10-digit number between 1,000,000,000 (September 8, 2001) and 2,000,000,000 (May 18, 2033) is almost certainly a Unix timestamp in seconds representing a recent date. A 13-digit number in the range 1,000,000,000,000 to 2,000,000,000,000 is almost certainly a JavaScript-style millisecond timestamp. Numbers outside these ranges could be timestamps for other epochs (Windows FILETIME uses January 1, 1601), very old or very future dates, or simply not timestamps at all. Context (column name, API documentation) is more reliable than range inference alone.
Is the Timestamp Converter free? Yes — completely free, no sign-up required.
Try the Timestamp Converter free at sadiqbd.com — convert any Unix timestamp to a human-readable date and time instantly.
