1.\" Copyright (c) 1989, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" Arthur Olson. 6.\" %sccs.include.redist.man% 7.\" 8.\" @(#)ctime.3 6.16 (Berkeley) 03/29/92 9.\" 10.Dd 11.Dt CTIME 3 12.Os BSD 4.3 13.Sh NAME 14.Nm asctime , 15.Nm ctime , 16.Nm difftime , 17.Nm gmtime , 18.Nm localtime , 19.Nm mktime 20.Nd transform binary date and time value to 21.Tn ASCII 22.Sh SYNOPSIS 23.Fd #include <sys/types.h> 24.Fd #include <time.h> 25.Vt extern char *tzname[2]; 26.Ft char * 27.Fn ctime "const time_t *clock" 28.Ft double 29.Fn difftime "time_t time1" "time_t time0" 30.Ft char * 31.Fn asctime "const struct tm *tm" 32.Ft struct tm * 33.Fn localtime "const time_t *clock" 34.Ft struct tm * 35.Fn gmtime "const time_t *clock" 36.Ft time_t 37.Fn mktime "struct tm *tm" 38.Sh DESCRIPTION 39The functions 40.Fn ctime , 41.Fn gmtime 42and 43.Fn localtime 44all take as an argument a time value representing the time in seconds since 45the Epoch (00:00:00 46.Tn UTC , 47January 1, 1970; see 48.Xr time 3 ) . 49.Pp 50The function 51.Fn localtime 52converts the time value pointed at by 53.Fa clock , 54and returns a pointer to a 55.Dq Fa struct tm 56(described below) which contains 57the broken-out time information for the value after adjusting for the current 58time zone (and any other factors such as Daylight Saving Time). 59Time zone adjustments are performed as specified by the 60.Ev TZ 61environmental variable (see 62.Xr tzset 3 ) . 63The function 64.Fn localtime 65uses 66.Xr tzset 67to initialize time conversion information if 68.Xr tzset 69has not already been called by the process. 70.Pp 71After filling in the tm structure, 72.Fn localtime 73sets the 74.Fa tm_isdst Ns 'th 75element of 76.Fa tzname 77to a pointer to an 78.Tn ASCII 79string that's the time zone abbreviation to be 80used with 81.Fn localtime Ns 's 82return value. 83.Pp 84The function 85.Fn gmtime 86similarly converts the time value, but without any time zone adjustment, 87and returns a pointer to a tm structure (described below). 88.Pp 89The 90.Fn ctime 91function 92adjusts the time value for the current time zone in the same manner as 93.Fn localtime , 94and returns a pointer to a 26-character string of the form: 95.Bd -literal -offset indent 96Thu Nov 24 18:22:48 1986\en\e0 97.Ed 98.Pp 99All the fields have constant width. 100.Pp 101The 102.Fn asctime 103function 104converts the broken down time in the structure 105.Fa tm 106pointed at by 107.Fa *tm 108to the form 109shown in the example above. 110.Pp 111The function 112.Fn mktime 113converts the broken-down time, expressed as local time, in the structure 114pointed to by tm into a time value with the same encoding as that of the 115values returned by the 116.Xr time 3 117function, that is, seconds from the Epoch, 118.Tn UTC . 119.Pp 120The original values of the 121.Fa tm_wday 122and 123.Fa tm_yday 124components of the structure are ignored, and the original values of the 125other components are not restricted to their normal ranges. 126(A positive or zero value for 127.Fa tm_isdst 128causes 129.Fn mktime 130to presume initially that summer time (for example, Daylight Saving Time) 131is or is not in effect for the specified time, respectively. 132A negative value for 133.Fa tm_isdst 134causes the 135.Fn mktime 136function to attempt to divine whether summer time is in effect for the 137specified time.) 138.Pp 139On successful completion, the values of the 140.Fa tm_wday 141and 142.Fa tm_yday 143components of the structure are set appropriately, and the other components 144are set to represent the specified calendar time, but with their values 145forced to their normal ranges; the final value of 146.Fa tm_mday 147is not set until 148.Fa tm_mon 149and 150.Fa tm_year 151are determined. 152.Fn Mktime 153returns the specified calendar time; if the calendar time cannot be 154represented, it returns \-1; 155.Pp 156The 157.Fn difftime 158function 159returns the difference between two calendar times, 160.Pf ( Fa time1 161- 162.Fa time0 ) , 163expressed in seconds. 164.Pp 165External declarations as well as the tm structure definition are in the 166.Aq Pa time.h 167include file. 168The tm structure includes at least the following fields: 169.Bd -literal -offset indent 170int tm_sec; /\(** seconds (0 - 60) \(**/ 171int tm_min; /\(** minutes (0 - 59) \(**/ 172int tm_hour; /\(** hours (0 - 23) \(**/ 173int tm_mday; /\(** day of month (1 - 31) \(**/ 174int tm_mon; /\(** month of year (0 - 11) \(**/ 175int tm_year; /\(** year \- 1900 \(**/ 176int tm_wday; /\(** day of week (Sunday = 0) \(**/ 177int tm_yday; /\(** day of year (0 - 365) \(**/ 178int tm_isdst; /\(** is summer time in effect? \(**/ 179char \(**tm_zone; /\(** abbreviation of timezone name \(**/ 180long tm_gmtoff; /\(** offset from UTC in seconds \(**/ 181.Ed 182.Pp 183The 184field 185.Fa tm_isdst 186is non-zero if summer time is in effect. 187.Pp 188The field 189.Fa tm_gmtoff 190is the offset (in seconds) of the time represented from 191.Tn UTC , 192with positive 193values indicating east of the Prime Meridian. 194.Sh SEE ALSO 195.Xr date 1 , 196.Xr gettimeofday 2 , 197.Xr getenv 3 , 198.Xr time 3 , 199.Xr tzset 3 , 200.Xr tzfile 5 201.Sh HISTORY 202This manual page is derived from 203the time package contributed to Berkeley by 204Arthur Olsen and which appeared in 205.Bx 4.3 . 206.Sh BUGS 207Except for 208.Fn difftime 209and 210.Fn mktime , 211these functions leaves their result in an internal static object and return 212a pointer to that object. Subsequent calls to these 213function will modify the same object. 214.Pp 215The 216.Fa tm_zone 217field of a returned tm structure points to a static array of characters, 218which will also be overwritten by any subsequent calls (as well as by 219subsequent calls to 220.Xr tzset 3 221and 222.Xr tzsetwall 3 ) . 223.Pp 224Use of the external variable 225.Fa tzname 226is discouraged; the 227.Fa tm_zone 228entry in the tm structure is preferred. 229.Pp 230Avoid using out-of-range values with 231.Fn mktime 232when setting up lunch with promptness sticklers in Riyadh. 233