This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding Linux
manual page for details of Linux behavior), or the interface may not be
implemented on Linux.
time_t time(time_t *tloc);
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of
POSIX.1‐2017 defers to the ISO C standard.
The time() function shall return the value of time in
seconds since the Epoch.
The tloc argument points to an area where the return value
is also stored. If tloc is a null pointer, no value is stored.
Upon successful completion, time() shall return the value of time.
Otherwise, (time_t)-1 shall be returned.
The time() function may fail if:
- The number of seconds since the Epoch will not fit in an object of type
The following sections are informative.
The following example uses the time() function to calculate the time
elapsed, in seconds, since the Epoch, localtime() to convert that value
to a broken-down time, and asctime() to convert the broken-down time
values into a printable string.
result = time(NULL);
printf("%s%ju secs since the Epoch\n",
This example writes the current time to stdout in a form
The following example gets the current time, prints it out in the user's format,
and prints the number of minutes to an event being timed.
Wed Jun 26 10:32:15 1996
835810335 secs since the Epoch
The time() function returns a value in seconds while
clock_gettime() and gettimeofday() return a struct
timespec (seconds and nanoseconds) and struct timeval (seconds and
microseconds), respectively, and are therefore capable of returning more
precise times. The times() function is also capable of more precision
than time() as it returns a value in clock ticks, although it returns
the elapsed time since an arbitrary point such as system boot time, not since
minutes_to_event = ...;
printf("The time is ");
printf("There are %d minutes to the event.\n",
Implementations in which time_t is a 32-bit signed integer
(many historical implementations) fail in the year 2038. POSIX.1‐2008
does not address this problem. However, the use of the time_t type is
mandated in order to ease the eventual fix.
On some systems the time() function is implemented using a
system call that does not return an error condition in addition to the
return value. On these systems it is impossible to differentiate between
valid and invalid return values and hence overflow conditions cannot be
The use of the <time.h> header instead of
<sys/types.h> allows compatibility with the ISO C
Many historical implementations (including Version 7) and the 1984
/usr/group standard use long instead of time_t. This volume of
POSIX.1‐2017 uses the latter type in order to agree with the
ISO C standard.
In a future version of this volume of POSIX.1‐2017, time_t is
likely to be required to be capable of representing times far in the future.
Whether this will be mandated as a 64-bit type or a requirement that a
specific date in the future be representable (for example, 10000 AD) is not
yet determined. Systems purchased after the approval of this volume of
POSIX.1‐2017 should be evaluated to determine whether their lifetime
will extend past 2038.
asctime(), clock(), clock_getres(),
ctime(), difftime(), futimens(),
localtime(), mktime(), strftime(),
strptime(), times(), utime()
The Base Definitions volume of POSIX.1‐2017,
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1-2017, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018
Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html .
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