1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd June 19, 2026 29.Dt ADJTIME 2 30.Os 31.Sh NAME 32.Nm adjtime 33.Nd "correct the time to allow synchronization of the system clock" 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In sys/time.h 38.Ft int 39.Fn adjtime "const struct timeval *delta" "struct timeval *olddelta" 40.Sh DESCRIPTION 41The 42.Fn adjtime 43system call 44makes small adjustments to the system time, as returned by 45.Xr gettimeofday 2 , 46advancing or retarding it 47by the time specified by the timeval 48.Fa delta . 49.Pp 50If 51.Fa delta 52is negative, the clock is 53slowed down by incrementing it more slowly than normal until 54the correction is complete. 55If 56.Fa delta 57is positive, a larger increment than normal 58is used. 59.Fa delta 60may also be a 61.Dv NULL 62pointer to fetch 63.Fa olddelta 64without making any adjustment. 65.Pp 66The skew used to perform the correction is generally a fraction of one percent. 67Thus, the time is always 68a monotonically increasing function. 69A time correction from an earlier call to 70.Fn adjtime 71may not be finished when 72.Fn adjtime 73is called again. 74If 75.Fa olddelta 76is not a 77.Dv NULL 78pointer, 79the structure pointed to will contain, upon return, the 80number of microseconds still to be corrected 81from the earlier call. 82.Pp 83This call may be used by time servers that synchronize the clocks 84of computers in a local area network. 85Such time servers would slow down the clocks of some machines 86and speed up the clocks of others to bring them to the average network time. 87.Pp 88The 89.Fn adjtime 90system call 91is restricted to the super-user. 92.Sh RETURN VALUES 93.Rv -std adjtime 94.Sh ERRORS 95The 96.Fn adjtime 97system call will fail if: 98.Bl -tag -width Er 99.It Bq Er EFAULT 100An argument points outside the process's allocated address space. 101.It Bq Er EPERM 102The process's effective user ID is not that of the super-user. 103.El 104.Sh SEE ALSO 105.Xr date 1 , 106.Xr gettimeofday 2 107.Rs 108.%T "TSP: The Time Synchronization Protocol for UNIX 4.3BSD" 109.%A R. Gusella 110.%A S. Zatti 111.Re 112.Sh HISTORY 113The 114.Fn adjtime 115system call appeared in 116.Bx 4.3 . 117