Preliminary ASN1_TIME documentation.

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=head1 NAME

ASN1_TIME_set, ASN1_TIME_adj, ASN1_TIME_check, ASN1_TIME_set_string,

ASN1_TIME_print, ASN1_TIME_diff - ASN.1 Time functions.

=head1 SYNOPSIS

 ASN1_TIME *ASN1_TIME_set(ASN1_TIME *s, time_t t);

 ASN1_TIME *ASN1_TIME_adj(ASN1_TIME *s, time_t t,

                          int offset_day, long offset_sec);

 int ASN1_TIME_set_string(ASN1_TIME *s, const char *str);

 int ASN1_TIME_check(const ASN1_TIME *t);

 int ASN1_TIME_print(BIO *b, const ASN1_TIME *s);

 int ASN1_TIME_diff(int *pday, int *psec,

                    const ASN1_TIME *from, const ASN1_TIME *to);

=head1 DESCRIPTION

The function ASN1_TIME_set() sets the ASN1_TIME structure B<s> to the

time represented by the time_t value B<t>. If B<s> is NULL a new ASN1_TIME

structure is allocated and returned.

ASN1_TIME_adj() sets the ASN1_TIME structure B<s> to the time represented

by the time B<offset_day> and B<offset_sec> after the time_t value B<t>.

The values of B<offset_day> or B<offset_sec> can be negative to set a

time before B<t>. The B<offset_sec> value can also exceed the number of

seconds in a day. If B<s> is NULL a new ASN1_TIME structure is allocated

and returned.

ASN1_TIME_set_string() sets ASN1_TIME structure B<s> to the time

represented by string B<str> which must be in appropriate ASN.1 time

format (for example YYMMDDHHMMSSZ or YYYYMMDDHHMMSSZ).

ASN1_TIME_check() checks the syntax of ASN1_TIME structure B<s>.

ASN1_TIME_print() prints out the time B<s> to BIO B<b> in human readable

format. It will be of the format MMM DD HH:MM:SS YYYY [GMT], for example

"Feb  3 00:55:52 2015 GMT" it does not include a newline. If the time

structure has invalid format it prints out "Bad time value" and returns

an error.

ASN1_TIME_diff() sets B<*pday> and B<*psec> to the time difference between

B<from> and B<to>. If B<to> represents a time later than B<from> then

one or both (depending on the time difference) of B<*pday> and B<*psec>

will be positive. If B<to> represents a time earlier than B<from> then

one or both of B<*pday> and B<*psec> will be negative. If B<to> and B<from>

represent the same time then B<*pday> and B<*psec> will both be zero.

If both B<*pday> and B<*psec> are non-zero they will always have the same

sign. The value of B<*psec> will always be less than the number of seconds

in a day. If B<from> or B<to> is NULL the current time is used.

=head1 NOTES

The ASN1_TIME structure corresponds to the ASN.1 structure B<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 structure is 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).

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 B<offset_sec> and B<offset_day> parameters instead of directly

manipulating a time_t value.

=head1 BUGS

ASN1_TIME_print() currently does not print out the time zone: it either prints

out "GMT" or nothing. But all certificates complying with RFC5280 et al use GMT

anyway.

=head1 EXAMPLES

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);

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");

=head1 RETURN VALUES

ASN1_TIME_set() and ASN1_TIME_adj() return a pointer to an ASN1_TIME structure

or NULL if an error occurred.

ASN1_TIME_set_string() returns 1 if the time value is successfully set and

0 otherwise.

ASN1_TIME_check() returns 1 if the structure is syntactically correct and 0

otherwise.

ASN1_TIME_print() returns 1 if the time is successfully printed out and 0 if

an error occurred (I/O error or invalid time format).

ASN1_TIME_diff() returns 1 for sucess and 0 for failure. It can fail if the

pass ASN1_TIME structure has invalid syntax for example.

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