ASN1_TIME_SET(3ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

okay bestie, welcome to the ASN1_TIME_SET(3ossl) manual page — the OpenSSL ASN.1 Time situation, part 1 of 2 🫶✨ this is a whole cast of functions and they are all kind of iconic in their own way 💅

🌸 NAME 🌸

This page is the home base for all the ASN.1 Time functions: ASN1_TIME_set, ASN1_UTCTIME_set, ASN1_GENERALIZEDTIME_set, ASN1_TIME_adj, ASN1_UTCTIME_adj, ASN1_GENERALIZEDTIME_adj, ASN1_TIME_check, ASN1_UTCTIME_check, ASN1_GENERALIZEDTIME_check, ASN1_TIME_set_string, ASN1_UTCTIME_set_string, ASN1_GENERALIZEDTIME_set_string, ASN1_TIME_set_string_X509, ASN1_TIME_normalize, ASN1_TIME_to_tm, ASN1_TIME_print, ASN1_TIME_print_ex, ASN1_UTCTIME_print, ASN1_GENERALIZEDTIME_print, ASN1_TIME_diff, ASN1_TIME_cmp_time_t, ASN1_UTCTIME_cmp_time_t, ASN1_TIME_compare, ASN1_TIME_to_generalizedtime, ASN1_TIME_dup, ASN1_UTCTIME_dup, and ASN1_GENERALIZEDTIME_dup — aka the ASN.1 Time functions, no cap 🎀

✨ SYNOPSIS ✨

here’s the whole squad in signature form, copy-paste exact because yes:

        ASN1_TIME *ASN1_TIME_set(ASN1_TIME *s, time_t t);
        ASN1_UTCTIME *ASN1_UTCTIME_set(ASN1_UTCTIME *s, time_t t);
        ASN1_GENERALIZEDTIME *ASN1_GENERALIZEDTIME_set(ASN1_GENERALIZEDTIME *s,
                                                       time_t t);

        ASN1_TIME *ASN1_TIME_adj(ASN1_TIME *s, time_t t, int offset_day,
                                 long offset_sec);
        ASN1_UTCTIME *ASN1_UTCTIME_adj(ASN1_UTCTIME *s, time_t t,
                                       int offset_day, long offset_sec);
        ASN1_GENERALIZEDTIME *ASN1_GENERALIZEDTIME_adj(ASN1_GENERALIZEDTIME *s,
                                                       time_t t, int offset_day,
                                                       long offset_sec);

        int ASN1_TIME_set_string(ASN1_TIME *s, const char *str);
        int ASN1_TIME_set_string_X509(ASN1_TIME *s, const char *str);
        int ASN1_UTCTIME_set_string(ASN1_UTCTIME *s, const char *str);
        int ASN1_GENERALIZEDTIME_set_string(ASN1_GENERALIZEDTIME *s,
                                            const char *str);

        int ASN1_TIME_normalize(ASN1_TIME *s);

        int ASN1_TIME_check(const ASN1_TIME *t);
        int ASN1_UTCTIME_check(const ASN1_UTCTIME *t);
        int ASN1_GENERALIZEDTIME_check(const ASN1_GENERALIZEDTIME *t);

        int ASN1_TIME_print(BIO *b, const ASN1_TIME *s);
        int ASN1_TIME_print_ex(BIO *bp, const ASN1_TIME *tm, unsigned long flags);
        int ASN1_UTCTIME_print(BIO *b, const ASN1_UTCTIME *s);
        int ASN1_GENERALIZEDTIME_print(BIO *b, const ASN1_GENERALIZEDTIME *s);

        int ASN1_TIME_to_tm(const ASN1_TIME *s, struct tm *tm);
        int ASN1_TIME_diff(int *pday, int *psec, const ASN1_TIME *from,
                           const ASN1_TIME *to);

        int ASN1_TIME_cmp_time_t(const ASN1_TIME *s, time_t t);
        int ASN1_UTCTIME_cmp_time_t(const ASN1_UTCTIME *s, time_t t);

        int ASN1_TIME_compare(const ASN1_TIME *a, const ASN1_TIME *b);

        ASN1_GENERALIZEDTIME *ASN1_TIME_to_generalizedtime(ASN1_TIME *t,
                                                           ASN1_GENERALIZEDTIME **out);

        ASN1_TIME *ASN1_TIME_dup(const ASN1_TIME *t);
        ASN1_UTCTIME *ASN1_UTCTIME_dup(const ASN1_UTCTIME *t);
        ASN1_GENERALIZEDTIME *ASN1_GENERALIZEDTIME_dup(const ASN1_GENERALIZEDTIME *t);

💖 DESCRIPTION 💖

okay so here’s the vibe check on every little group of functions, bestie 🌈

  • ASN1_TIME_set(), ASN1_UTCTIME_set(), ASN1_GENERALIZEDTIME_set() — these set the structure s to the time represented by the time_t value t. If s is NULL, a new time structure is allocated and returned. slay ✨
  • ASN1_TIME_adj(), ASN1_UTCTIME_adj(), ASN1_GENERALIZEDTIME_adj() — these set the time structure s to the time represented by the time offset_day and offset_sec after the time_t value t. The values of offset_day or offset_sec can be negative to set a time before t. The offset_sec value can also exceed the number of seconds in a day. If s is NULL, a new structure is allocated and returned 👑
  • ASN1_TIME_set_string(), ASN1_UTCTIME_set_string(), ASN1_GENERALIZEDTIME_set_string() — these set the time structure s to the time represented by string str, which must be in appropriate ASN.1 time format (for example YYMMDDHHMMSSZ or YYYYMMDDHHMMSSZ). If s is NULL, this function performs a format check on str only. The string str is copied into s 🎀
  • ASN1_TIME_set_string_X509() — sets ASN1_TIME structure s to the time represented by string str, which must be in appropriate time format that RFC 5280 requires. That means it only allows YYMMDDHHMMSSZ and YYYYMMDDHHMMSSZ (leap second is rejected); all other ASN.1 time format are not allowed. If s is NULL, this function performs a format check on str only 🦋
  • ASN1_TIME_normalize() — converts an ASN1_GENERALIZEDTIME or ASN1_UTCTIME into a time value that can be used in a certificate. It should be used after the ASN1_TIME_set_string() functions and before ASN1_TIME_print() functions to get consistent (i.e. GMT) results 🌸
  • ASN1_TIME_check(), ASN1_UTCTIME_check(), ASN1_GENERALIZEDTIME_check() — these check the syntax of the time structure s 🧚
  • ASN1_TIME_print(), ASN1_UTCTIME_print(), ASN1_GENERALIZEDTIME_print() — these print the time structure s to BIO b in human readable format. It will be of the format MMM DD HH:MM:SS[.s*] YYYY GMT, for example "Feb 3 00:55:52 2015 GMT", which does not include a newline. If the time structure has invalid format it prints out "Bad time value" and returns an error. The output for generalized time may include a fractional part following the second 💜
  • ASN1_TIME_print_ex() — provides flags to specify the output format of the datetime. This can be either ASN1_DTFLGS_RFC822 or ASN1_DTFLGS_ISO8601 ⭐
  • ASN1_TIME_to_tm() — converts the time s to the standard tm structure. If s is NULL, then the current time is converted. The output time is GMT. The tm_sec, tm_min, tm_hour, tm_mday, tm_wday, tm_yday, tm_mon and tm_year fields of the tm structure are set to proper values, whereas all other fields are set to 0. If tm is NULL, this function performs a format check on s only. If s is in Generalized format with fractional seconds, e.g. YYYYMMDDHHMMSS.SSSZ, the fractional seconds will be lost while converting s to tm structure 😭
  • ASN1_TIME_diff() — sets *pday and *psec to the time difference between from and to. If to represents a time later than from, then one or both (depending on the time difference) of *pday and *psec will be positive. If to represents a time earlier than from, then one or both of *pday and *psec will be negative. If to and from represent the same time, then *pday and *psec will both be zero. If both *pday and *psec are nonzero they will always have the same sign. The value of *psec will always be less than the number of seconds in a day. If from or to is NULL, the current time is used 🔥
  • ASN1_TIME_cmp_time_t() and ASN1_UTCTIME_cmp_time_t() — these compare the two times represented by the time structure s and the time_t t 💫
  • ASN1_TIME_compare() — compares the two times represented by the time structures a and b 🫶
  • ASN1_TIME_to_generalizedtime() — converts an ASN1_TIME to an ASN1_GENERALIZEDTIME, regardless of year. If either out or *out are NULL, then a new object is allocated and must be freed after use 🍓
  • ASN1_TIME_dup(), ASN1_UTCTIME_dup(), ASN1_GENERALIZEDTIME_dup() — these duplicate the time structure t and return the duplicated result correspondingly 👑

🦋 NOTES 🦋

  • The ASN1_TIME structure corresponds to the ASN.1 structure Time defined in RFC5280 et al. The time setting functions obey the rules outlined in RFC5280: if the date can be represented by UTCTime it is used, else GeneralizedTime is used ✨
  • The ASN1_TIME, ASN1_UTCTIME and ASN1_GENERALIZEDTIME structures are represented as an ASN1_STRING internally and can be freed up using ASN1_STRING_free() 🎀
  • The ASN1_TIME structure can represent years from 0000 to 9999 but no attempt is made to correct ancient calendar changes (for example from Julian to Gregorian calendars) 🌸
  • ASN1_UTCTIME is limited to a year range of 1950 through 2049 💜
  • Some applications add offset times directly to a time_t value and pass the results to ASN1_TIME_set() (or equivalent). This can cause problems as the time_t value can overflow on some systems resulting in unexpected results. New applications should use ASN1_TIME_adj() instead and pass the offset value in the offset_sec and offset_day parameters instead of directly manipulating a time_t value ⚠️
  • ASN1_TIME_adj() may change the type from ASN1_GENERALIZEDTIME to ASN1_UTCTIME, or vice versa, based on the resulting year. ASN1_GENERALIZEDTIME_adj() and ASN1_UTCTIME_adj() will not modify the type of the return structure 🌈
  • It is recommended that functions starting with ASN1_TIME be used instead of those starting with ASN1_UTCTIME or ASN1_GENERALIZEDTIME. The functions starting with ASN1_UTCTIME and ASN1_GENERALIZEDTIME act only on that specific time format. The functions starting with ASN1_TIME will operate on either format 💅
  • Users familiar with RFC822 should note that when specifying the flag ASN1_DTFLGS_RFC822 the year will be formatted as documented above, i.e., using 4 digits, not 2 as specified in RFC822 🧚

💀 BUGS 💀

  • ASN1_TIME_print(), ASN1_UTCTIME_print() and ASN1_GENERALIZEDTIME_print() do not print out the timezone: it either prints out "GMT" or nothing. But all certificates complying with RFC5280 et al use GMT anyway 🙈
  • ASN1_TIME_print(), ASN1_TIME_print_ex(), ASN1_UTCTIME_print() and ASN1_GENERALIZEDTIME_print() do not distinguish if they fail because of an I/O error or invalid time format 😭
  • Use the ASN1_TIME_normalize() function to normalize the time value before printing to get GMT results 🫶

🌟 RETURN VALUES 🌟

  • ASN1_TIME_set(), ASN1_UTCTIME_set(), ASN1_GENERALIZEDTIME_set(), ASN1_TIME_adj(), ASN1_UTCTIME_adj() and ASN1_GENERALIZEDTIME_set() return a pointer to a time structure or NULL if an error occurred ✨
  • ASN1_TIME_set_string(), ASN1_UTCTIME_set_string(), ASN1_GENERALIZEDTIME_set_string() and ASN1_TIME_set_string_X509() return 1 if the time value is successfully set and 0 otherwise 🎀
  • ASN1_TIME_normalize() returns 1 on success, and 0 on error 💜
  • ASN1_TIME_check(), ASN1_UTCTIME_check and ASN1_GENERALIZEDTIME_check() return 1 if the structure is syntactically correct and 0 otherwise 🌸
  • ASN1_TIME_print(), ASN1_UTCTIME_print() and ASN1_GENERALIZEDTIME_print() return 1 if the time is successfully printed out and 0 if an I/O error occurred an error occurred (I/O error or invalid time format) 😭
  • ASN1_TIME_to_tm() returns 1 if the time is successfully parsed and 0 if an error occurred (invalid time format) 🦋
  • ASN1_TIME_diff() returns 1 for success and 0 for failure. It can fail if the passed-in time structure has invalid syntax, for example 🔥
  • ASN1_TIME_cmp_time_t() and ASN1_UTCTIME_cmp_time_t() return -1 if s is before t, 0 if s equals t, or 1 if s is after t. -2 is returned on error 👑
  • ASN1_TIME_compare() returns -1 if a is before b, 0 if a equals b, or 1 if a is after b. -2 is returned on error 💫
  • ASN1_TIME_to_generalizedtime() returns a pointer to the appropriate time structure on success or NULL if an error occurred 🍓
  • ASN1_TIME_dup(), ASN1_UTCTIME_dup() and ASN1_GENERALIZEDTIME_dup() return a pointer to a time structure or NULL if an error occurred 🌈

✨ EXAMPLES ✨

Okay bestie, real talk — this is where it gets fun. Here's how you actually use these things 💖

Set a time to one hour from now and print it out

Lowkey the cleanest little demo ever. Set a time structure to one hour after the current time and print it out:

#include <time.h>
#include <openssl/asn1.h>

ASN1_TIME *tm;
time_t t;
BIO *b;

t = time(NULL);
tm = ASN1_TIME_adj(NULL, t, 0, 60 * 60);
b = BIO_new_fp(stdout, BIO_NOCLOSE);
ASN1_TIME_print(b, tm);
ASN1_STRING_free(tm);
BIO_free(b);

Notice that 60 * 60? That's sixty minutes times sixty seconds, aka one hour, and yes we're doing math in public 🧮✨ Calling ASN1_TIME_adj() with NULL as the first arg means it allocates a shiny new ASN1_TIME for you, and then you gotta ASN1_STRING_free() it when you're done because we do not leave memory leaks lying around 💅

Is this time later or sooner than now?

Determine if one time is later or sooner than the current time:

int day, sec;

if (!ASN1_TIME_diff(&day, &sec, NULL, to))
    /* Invalid time format */

if (day > 0 || sec > 0)
    printf("Later\n");
else if (day < 0 || sec < 0)
    printf("Sooner\n");
else
    printf("Same\n");

Passing NULL as the from argument means "compare against right now", and the diff gets poured into day and sec 🫶 Also note the vibe check in that if (!ASN1_TIME_diff(...)) — if it returns false you're dealing with an invalid time format, so handle that before you go printing "Later" like a liar 😭

💜 HISTORY 💜

Not me giving you the lore drop 📜 The ASN1_TIME_to_tm() function was added in OpenSSL 1.1.1.

The ASN1_TIME_set_string_X509() function was added in OpenSSL 1.1.1.

The ASN1_TIME_normalize() function was added in OpenSSL 1.1.1.

The ASN1_TIME_cmp_time_t() function was added in OpenSSL 1.1.1.

The ASN1_TIME_compare() function was added in OpenSSL 1.1.1.

Bestie that's five functions all dropping in 1.1.1 — that release really said "I'll do it myself" and ate 👑🔥

Copyright 2015-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

Fr fr, read the License before you go copying this around — "not in compliance" is not the flex you think it is 🙈

Okay that's the whole page, you made it to the end, I'm so proud of you 🎀✨ This gem is from OpenSSL 3.5.6, dated 2026-04-07, and it's filed under ASN1_TIME_SET(3ossl) — go forth and handle your timestamps responsibly, mwah 💋🌸

📦 where this page comes from