Preliminary ASN1_TIME documentation.

Reviewed-by: Rich Salz <rsalz@openssl.org>

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+=pod
+
+=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.
+
+=cut