TZSET(3POSIX)

💖 TZSET(3POSIX) 💖

okay bestie, welcome to the POSIX Programmer's Manual — yes, that one. This is where we talk about daylight, timezone, tzname and tzset. Buckle up, it's giving timezone lore ✨

🌸 PROLOG 🌸

so like, real talk first: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (if you're on Linux, go check the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not be implemented on Linux at all. no cap, read the fine print 🫶

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👑 NAME 👑

daylight, timezone, tzname, tzset — set timezone conversion information. that's the whole job, that's the assignment 💅

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✨ SYNOPSIS ✨

#include <time.h>

extern int daylight;
extern long timezone;
extern char *tzname[2];
void tzset(void);

like, four little lines and a whole personality. iconic.

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🦋 DESCRIPTION 🦋

okay so here's the tea. The tzset() function shall use the value of the environment variable TZ to set time conversion information used by ctime(), localtime(), mktime(), and strftime(). If TZ is absent from the environment, implementation-defined default timezone information shall be used. (translation: no TZ, no say, the implementation picks for you 🙈)

The tzset() function shall set the external variable tzname as follows:

tzname[0] = "std";
tzname[1] = "dst";

where std and dst are as described in the Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables. iykyk 📖

The tzset() function also shall set the external variable daylight to 0 if Daylight Savings Time conversions should never be applied for the timezone in use; otherwise, non-zero. The external variable timezone shall be set to the difference, in seconds, between Coordinated Universal Time (UTC) and local standard time. it's literally just doing math for you, main character energy fr fr ⭐

⚠️ okay this part is IMPORTANT bestie, do not skip: If a thread accesses tzname, daylight, or timezone directly while another thread is in a call to tzset(), or to any function that is required or allowed to set timezone information as if by calling tzset(), the behavior is undefined. undefined behavior is not a vibe. do not do this. 💀

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🎀 RETURN VALUE 🎀

The tzset() function shall not return a value. it's a void girlie. she gives nothing back and she's thriving 🌸

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🌈 ERRORS 🌈

No errors are defined. zero drama, we love that for her.

The following sections are informative.

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💜 EXAMPLES 💜

Example TZ variables and their timezone differences are given in the table below (rip the pretty table, here it is as a list, same facts, cuter font):

• EST5EDT → timezone = 5*60*60
• GMT0 → timezone = 0*60*60
• JST-9 → timezone = -9*60*60
• MET-1MEST → timezone = -1*60*60
• MST7MDT → timezone = 7*60*60
• PST8PDT → timezone = 8*60*60

look at all those little offsets living their best lives 🍓

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🫶 APPLICATION USAGE 🫶

Since the ctime(), localtime(), mktime(), strftime(), and strftime_l() functions are required to set timezone information as if by calling tzset(), there is no need for an explicit tzset() call before using these functions. they already did it for you, no need to double text 📱

However, portable applications should call tzset() explicitly before using ctime_r() or localtime_r() because setting timezone information is optional for those functions. so if you're writing portable code, call it yourself. be the responsible one 💅

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🧚 RATIONALE 🧚

None. she said nothing. silence. queen behavior.

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💫 FUTURE DIRECTIONS 💫

None. no future plans, we're living in the now, periodt.

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🔥 SEE ALSO 🔥

• ctime()
• localtime()
• mktime()
• strftime()

The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, <time.h>

go read those too bestie, they're all in the same friend group 💖

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📜 COPYRIGHT 📜

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 .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, babe ✨ this one came to you from IEEE/The Open Group, dated 2017, and honestly? they ate. now go set your TZ and slay 💖🦋🎀