Unix time
Unix time, or POSIX time, is a system for describing points in time. It is widely used not only on Unix-like operating systems but in many other computing systems. It is an encoding of UTC, and is sufficiently similar to a linear representation of the passage of time that it is frequently mistaken for one.
Definition
There are two layers of encoding that make up Unix time, and they can be usefully separated. The first layer encodes a point in time as a scalar real number, and the second encodes that number as a sequence of bits or in some other manner.
Encoding time as a number
Modern Unix time is based strictly on UTC. UTC counts time using SI seconds, and breaks up the span of time into days. UTC days are mostly 86400 s long, but are occasionally 86401 s or 86399 s in order to keep the days synchronised with the rotation of the Earth. As is standard with UTC, this article will label days using the Gregorian calendar, and count times within each day in hours, minutes, and seconds. Some of the examples will also show TAI, another time scheme, which uses the same seconds and is displayed in the same format as UTC but makes no attempt to stay synchronised with the Earth's rotation.
The Unix epoch is the time 00:00:00 UTC on January 1 1970. There is a problem with this definition, in that UTC didn't exist in its current form until 1972; this issue is discussed below. For brevity, the remainder of this article will use ISO 8601 date format, in which the Unix epoch is 1970-01-01T00:00:00Z.
The Unix time number is zero at the Unix epoch, and increases by exactly 86400 per day since the epoch. Thus 2004-09-16T00:00:00Z, 12677 days after the epoch, is represented by the Unix time number 12677 * 86400 = 1095292800. This can be extended backwards from the epoch too, using negative numbers; thus 1957-10-04T00:00:00Z, 4472 days before the epoch, is represented by the Unix time number -4472 * 86400 = -386380800.
Within each day, the Unix time number is as calculated in the preceding paragraph at midnight UTC (00:00:00Z), and increases by exactly 1 per second since midnight. Thus 2004-09-16T17:55:43.54Z, 64543.54 s since midnight on the day in the example above, is represented by the Unix time number 1095292800 + 64543.54 = 1095357343.54. On dates before the epoch the number still increases, thus becoming less negative, as time moves forward.
The above scheme means that on a normal UTC day, of duration 86400 s, the Unix time number changes in a continuous manner across midnight. For example, at the end of the day used in the examples above, the time representations progress like this:
| TAI | UTC | Unix time |
|---|---|---|
| 2004-09-17T00:00:30.75Z | 2004-09-16T23:59:58.75Z | 1095379198.75 |
| 2004-09-17T00:00:31.00Z | 2004-09-16T23:59:59.00Z | 1095379199.00 |
| 2004-09-17T00:00:31.25Z | 2004-09-16T23:59:59.25Z | 1095379199.25 |
| 2004-09-17T00:00:31.50Z | 2004-09-16T23:59:59.50Z | 1095379199.50 |
| 2004-09-17T00:00:31.75Z | 2004-09-16T23:59:59.75Z | 1095379199.75 |
| 2004-09-17T00:00:32.00Z | 2004-09-17T00:00:00.00Z | 1095379200.00 |
| 2004-09-17T00:00:32.25Z | 2004-09-17T00:00:00.25Z | 1095379200.25 |
| 2004-09-17T00:00:32.50Z | 2004-09-17T00:00:00.50Z | 1095379200.50 |
| 2004-09-17T00:00:32.75Z | 2004-09-17T00:00:00.75Z | 1095379200.75 |
| 2004-09-17T00:00:33.00Z | 2004-09-17T00:00:01.00Z | 1095379201.00 |
| 2004-09-17T00:00:33.25Z | 2004-09-17T00:00:01.25Z | 1095379201.25 |
When a leap second occurs, so that the UTC day is not exactly 86400 s long, a discontinuity occurs in the Unix time number. The Unix time number increases by exactly 86400 each day, regardless of how long the day is. When a leap second is deleted (which has never occurred, as of 2004), the Unix time number jumps up by 1 at the instant where the leap second was deleted from, which is the start of the next day. When a leap second is inserted (which occurs on average once every year and a half), the Unix time number increases continuously during the leap second, during which time it is more than 86400 s since the start of the current day, and then jumps down by 1 at the end of the leap second, which is the start of the next day. For example, this is what happened at the end of 1998:
| TAI | UTC | Unix time |
|---|---|---|
| 1999-01-01T00:00:29.75Z | 1998-12-31T23:59:58.75Z | 915148798.75 |
| 1999-01-01T00:00:30.00Z | 1998-12-31T23:59:59.00Z | 915148799.00 |
| 1999-01-01T00:00:30.25Z | 1998-12-31T23:59:59.25Z | 915148799.25 |
| 1999-01-01T00:00:30.50Z | 1998-12-31T23:59:59.50Z | 915148799.50 |
| 1999-01-01T00:00:30.75Z | 1998-12-31T23:59:59.75Z | 915148799.75 |
| 1999-01-01T00:00:31.00Z | 1998-12-31T23:59:60.00Z | 915148800.00 |
| 1999-01-01T00:00:31.25Z | 1998-12-31T23:59:60.25Z | 915148800.25 |
| 1999-01-01T00:00:31.50Z | 1998-12-31T23:59:60.50Z | 915148800.50 |
| 1999-01-01T00:00:31.75Z | 1998-12-31T23:59:60.75Z | 915148800.75 |
| 1999-01-01T00:00:32.00Z | 1999-01-01T00:00:00.00Z | 915148800.00 |
| 1999-01-01T00:00:32.25Z | 1999-01-01T00:00:00.25Z | 915148800.25 |
| 1999-01-01T00:00:32.50Z | 1999-01-01T00:00:00.50Z | 915148800.50 |
| 1999-01-01T00:00:32.75Z | 1999-01-01T00:00:00.75Z | 915148800.75 |
| 1999-01-01T00:00:33.00Z | 1999-01-01T00:00:01.00Z | 915148801.00 |
| 1999-01-01T00:00:33.25Z | 1999-01-01T00:00:01.25Z | 915148801.25 |
Observe that when a positive leap second occurs (i.e., when a leap second is inserted) the Unix time numbers repeat themselves. The Unix time number 915148800.50 is ambiguous: it can refer either to the instant in the middle of the leap second, or to the instant one second later, half a second after midnight UTC. In the theoretical case when a negative leap second occurs (i.e., when a leap second is deleted) no ambiguity is caused, but instead there is a range of Unix time numbers that do not refer to any point in time at all.
When dealing with time periods that do not encompass a UTC leap second, the difference between two Unix time numbers is equal to the duration in seconds of the time period between the corresponding points in time. This is a common computational technique. However, where leap seconds occur, such calculations give the wrong answer. In applications where this level of accuracy is required, it is necessary to consult a table of leap seconds when dealing with Unix times, and it is often preferable to use a different time encoding that does not suffer this problem.
A Unix time number is easily converted back into UTC by taking the quotient and modulus of the Unix time number, modulo 86400. The quotient is the number of days since the epoch, and the modulus is the number of seconds since midnight UTC on that day. (It is important to ensure that the right type of modulus is being calculated when dealing with times before the epoch.) If given a Unix time number that is ambiguous due to a positive leap second, this algorithm will interpret it as the time just after midnight. It will never generate a time that is during a leap second. If given a Unix time number that is invalid due to a negative leap second, it will generate an equally invalid UTC time. If these conditions are significant then it is necessary to consult a table of leap seconds in order to detect them.
Representing the number
A Unix time number can be represented in any form capable of representing numbers. In some applications the number is simply represented textually as a string of decimal digits, raising only trivial additional issues. However, there are certain binary representations of Unix times that are of particular significance.
The standard Unix time_t (data type representing a point in time) is a signed integer data type, of 32 bits or more, directly encoding the Unix time number as described in the preceding section. Being integer means that it has a resolution of one second; many Unix applications therefore handle time only to that resolution. Being 32 bits means that it covers a range of about 136 years in total. The minimum representable time is 1901-12-13T20:45:52Z, and the maximum representable time is 2038-01-19T03:14:07Z. At 2038-01-19T03:14:08Z this representation will overflow. This milestone is anticipated with a mixture of amusement and dread.
In some newer operating systems, time_t has been widened to 64 bits. In the negative direction, this goes back more than twenty times the age of the universe, and so suffices. In the positive direction, whether this range is sufficient to represent all possible times depends on the ultimate fate of the universe, but it can be expected to postpone overflow long enough for such implementation limits to be abolished.
There has been some controversy over whether the Unix time_t should be signed or unsigned. If unsigned, its range in the future would be doubled, postponing the 32 bit overflow. However, it would then be incapable of representing times prior to 1970. The consensus, and universal practice, is for time_t to be signed.
When representing times with sub-second precision, Unix has strangely little tradition of representing a non-integer time number directly. Instead, there are compound data types, usually known by their C names, that consist of two integers, the first being a time_t (the integral part of the Unix time), and the second being the fractional part of the time number in millionths (in struct timeval) or billionths (in struct timespec). These structures provide a decimal-based fixed-point data format, which is useful for some applications, but in others must be endured.
UTC basis
The present form of UTC, with leap seconds, is defined only from January 1 1972 onwards. Prior to that, since January 1 1961 there was an older form of UTC in which not only were there occasional time steps, which were by non-integer numbers of seconds, but also the UTC second was slightly longer than the SI second, and periodically changed, in order to continuously approximate the Earth's rotation. Prior to 1961 there was no UTC, and prior to 1958 there was no widespread atomic timekeeping; in these eras, some approximation of GMT (based directly on the Earth's rotation) was used instead of an atomic timescale.
The precise definition of Unix time as an encoding of UTC is only uncontroversially applicable to the present form of UTC. Fortunately, the fact that the Unix epoch predates the start of this form of UTC does not affect its use in this era: the number of days from January 1 1970 (the Unix epoch) to January 1 1972 (the start of UTC) is not in question, and the number of days is all that is significant to Unix time.
The meaning of Unix time values below +63072000 (i.e., prior to January 1 1972) is not precisely defined. The basis of such Unix times is best understood to be an unspecified approximation of GMT. Computers of that era rarely had clocks set sufficiently accurately to provide meaningful sub-second timestamps in any case. Unix time is not a suitable way to represent times prior to 1972 in applications requiring sub-second precision; such applications must, at least, define which form of UT or GMT they are using.
As of 2004, the possibility of ending the use of leap seconds in civil time is being considered. A likely means to execute this change is to define a new time scale, called "International Time", that initially matches UTC but thereafter has no leap seconds, thus remaining at a constant offset from TAI. If this happens, it is likely that Unix time will be prospectively defined in terms of this new time scale, instead of UTC. Uncertainty about whether this will occur makes prospective Unix time no less predictable than it already is: if UTC were simply to have no further leap seconds the result would be the same.
History
The earliest versions of Unix time had a 32 bit integer incrementing at a rate of 60 Hz, which was the rate of the system clock on the hardware of the early Unix systems. The value 60 Hz still appears in some software interfaces as a result. The epoch also differed from the current value. The first edition Unix Programmer's Manual dated November 3 1971 defines the Unix time as "the time since 00:00:00, Jan. 1, 1971, measured in sixtieths of a second". It also comments that "the chronologically-minded user will note that 2**32 sixtieths of a second is only about 2.5 years". Because of this limited range, the epoch was redefined more than once, before the rate was changed to 1 Hz (yielding a range in excess of 130 years) and the epoch was set to its present value.
As indicated by the definition quoted above, the Unix time scale was originally intended to be a simple linear representation of time elapsed since an epoch. However, there was no consideration of the details of time scales, and it was implicitly assumed that there was a simple linear time scale already available and agreed upon. Indeed, the first edition manual's definition doesn't even specify which timezone is used. Several later problems, including the complexity of the present definition, result from Unix time having been defined gradually by usage rather than fully defined to start with.
When POSIX.1 was written, in the 1980s (it was published in 1988, the question arose of how to precisely define time_t in the face of leap seconds. Some argued for it to remain, as intended, a linear count of seconds since the epoch, at the expense of complexity in conversions with civil time. Others argued for it to remain, as conflictingly intended, easily interconvertible with the conventional representation of civil time, at the expense of inconsistency around leap seconds. Computer clocks of the era were not sufficiently precisely set to form a precedent one way or the other. The POSIX committee was swayed by arguments against complexity in the library functions, and firmly defined the Unix time in a simple manner in terms of the elements of UTC time. Unfortunately, this definition was so simple that it didn't even encompass the entire leap year rule of the Gregorian calendar, and would make 2100 a leap year.
The 2001 edition of POSIX.1 rectified the faulty leap year rule in the definition of Unix time, but retained the essential definition of Unix time as an encoding of UTC rather than a linear time scale. Also, since the mid-1990s computer clocks have been routinely set with sufficient precision for this to matter, and they have most commonly been set using the UTC-based definition of Unix time. This has resulted in considerable complexity in Unix implementations, and in the NTP protocol, in order to execute steps in the Unix time number whenever leap seconds occur. Those who have to implement or interface to the gory details regularly curse the POSIX committee and the inventor of UTC, seeing the whole affair as a sordid mess.
As of 2004, POSIX has new interfaces making several different time scales available to programs, splitting up the many uses to which Unix times have traditionally been put. The future is one where time values are accompanied by explicit labels of the time scale defining their significance. Unix time as described in this article will still be in wide use for decades to come, but is likely to be increasingly treated as a legacy system and superseded by better-defined systems.