man3/timegm.3

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  25 .TH TIMEGM 3 2016-03-15 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 timegm, timelocal \- inverses of gmtime and localtime
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <time.h>
  31 .sp
  32 .BI "time_t timelocal(struct tm *" tm );
  33 .sp
  34 .BI "time_t timegm(struct tm *" tm );
  35 .sp
  36 .fi
  37 .in -4n
  38 Feature Test Macro Requirements for glibc (see
  39 .BR feature_test_macros (7)):
  40 .in
  41 .sp
  42 .BR timelocal (),
  43 .BR timegm ():
  44     Since glibc 2.19:
  45         _DEFAULT_SOURCE
  46     Glibc 2.19 and earlier:
  47         _BSD_SOURCE || _SVID_SOURCE
  48 .SH DESCRIPTION
  49 The functions
  50 .BR timelocal ()
  51 and
  52 .BR timegm ()
  53 are the inverses of
  54 .BR localtime (3)
  55 and
  56 .BR gmtime (3).
  57 Both functions take a broken-down time and convert it to calendar time
  58 (seconds since the Epoch, 1970-01-01 00:00:00 +0000, UTC).
  59 The difference between the two functions is that
  60 .BR timelocal ()
  61 takes the local timezone into account when doing the conversion, while
  62 .BR timegm ()
  63 takes the input value to be Coordinated Universal Time (UTC).
  64 .SH ATTRIBUTES
  65 For an explanation of the terms used in this section, see
  66 .BR attributes (7).
  67 .TS
  68 allbox;
  69 lbw21 lb lb
  70 l l l.
  71 Interface       Attribute       Value
  72 T{
  73 .BR timelocal (),
  74 .BR timegm ()
  75 T}      Thread safety   MT-Safe env locale
  76 .TE
  77 .SH CONFORMING TO
  78 These functions are nonstandard GNU extensions
  79 that are also present on the BSDs.
  80 Avoid their use.
  81 .SH NOTES
  82 The
  83 .BR timelocal ()
  84 function is equivalent to the POSIX standard function
  85 .BR mktime (3).
  86 There is no reason to ever use it.
  87 .SH SEE ALSO
  88 .BR gmtime (3),
  89 .BR localtime (3),
  90 .BR mktime (3),
  91 .BR tzset (3)